Method of Estimating

Quantities of a building (mainly walls and foundation) are taken out by the following methods.

1. Long wall and short wall method

• Walls running in the long direction are called long walls; those running in the short direction are short walls.
• Long wall length (out-to-out) = centre-to-centre length + half the width of the trench/wall at each end.
• Short wall length (in-to-in) = centre-to-centre length - half the width at each end.
• Each wall is multiplied by its own width and depth. It is accurate but needs more calculation; it suits walls of different thicknesses.

2. Centre line method

• The total centre-line length of all walls is found and multiplied by the section (width x depth) of the wall or foundation.
• Where walls meet (junctions), a deduction of half the width of the wall for each junction is made from the total length.
• It is quick and best for buildings with uniform walls and rooms of the same type.

3. Crossing (partition) wall method

• Separate cross-walls are measured by their own lengths and multiplied by the section; useful when walls differ in thickness.

4. Individual wall (separate wall) method

• Each wall is measured one by one.

5. Bay method

• The building is divided into bays between columns, and each bay is measured, used for framed structures and sheds.

6. Unit (approximate) methods

• Cost per unit area/volume for a quick estimate.

In practice, the long wall-short wall and centre line methods are the two used in building estimates in Nepal.

Bill of quantities (BOQ)

A bill of quantities is a document that lists, item by item, all the works of a project with their description, unit and quantity, derived from the drawings and specifications. In a priced BOQ the rate and amount for each item are added.

A typical BOQ has these columns:

The total amount (plus contingency and VAT) is the project cost.

Importance

1. Basis of tendering: all bidders price the same list of quantities, so bids can be compared fairly.
2. Cost estimate: the BOQ with rates gives the abstract of cost and the total estimate.
3. Contract document: in a unit-rate contract it is the basis of the contract price.
4. Payment: interim and final bills are prepared against the BOQ items; variations are valued at the BOQ rates.
5. Planning: gives quantities of materials, labour and plant for procurement and scheduling.
6. Cost control: actual quantities and costs are checked against the BOQ.
7. Variation valuation: additions and omissions can be priced easily.

A detailed estimate is prepared by dividing the building into items of work, taking out the quantities of each item from the drawings, and applying rates. The steps are:

1. Study the drawings and specifications: plan, section, elevation, foundation and structural drawings, and door/window schedules.
2. List the items in the order of construction: earthwork, PCC, foundation masonry/RCC, DPC, superstructure masonry, columns, beams, slab, doors and windows, plastering, flooring, painting, roofing, sanitary and electrical works.
3. Take out quantities of each item using the long wall-short wall or centre line method. Use the dimension columns: number x length x breadth x height, and make deductions for openings. A quantity sheet looks like this:

4. Abstract the quantities (collect similar items) in a bill of quantities.
5. Rate analysis: adopt rates from the District Rate or analyse them using DUDBC norms and local rates.
6. Abstract of cost: quantity x rate for each item gives the amount; sum of all items is the base cost.
7. Add provisions: contingency (3 to 5%), work-charged establishment/supervision, and VAT (13%) to get the total estimated cost.
8. Prepare the report: a brief report with the specifications, assumptions, drawings, and the cost summary.

Plastering is measured over the wall surface in m². For openings in brick surfaces, IS 1200 (Part 12) gives the following rules.

Other points:

• Where the plaster is on one face only, the opening is deducted from that face as per the same limits.
• For ends of walls, jambs, sills and soffits of larger openings, the plastering of the reveal is measured by width x girth and added.
• No deduction is made for the ends of beams, joists, or rafters, and for small holes.
• Many Nepali offices, for simplicity, deduct the full area of every opening on the plastered face and then add the reveals.

Example: a wall 4 m x 3 m has a window 1.2 m x 1.2 m (1.44 m²) and plaster on both faces. The deduction is 1.44 m² once (one face only), so the net plaster = 2 x 12 - 1.44 = 22.56 m².

A building estimate is divided into sub-heads (groups of similar items), arranged in the order of construction. This makes quantity taking and checking easy.

Finally, contingency, VAT and work-charged establishment are added to the total of all sub-heads.

Bill of quantities (BOQ)

The BOQ lists all items of work with their description, unit and quantity (and, once priced, rate and amount). It is used in tendering and as the basis of the contract and payment.

Abstract of cost (AOC)

The abstract of cost is a summary sheet in which the quantity of each item (from the BOQ or quantity sheet) is multiplied by its rate to obtain the amount; the total of all items, with contingency and taxes, gives the estimated cost of the project.

Relationship

• Both are built from the same quantity calculation sheets.
• The BOQ gives quantities; adding rates to the BOQ gives the AOC (a priced BOQ is, in effect, an abstract of cost).
• The totals of both must agree.

Differences

i) Plaster work

• Unit: m² (measurement and payment).
• Method: the length and height of the wall surface are measured on each plastered face (floor to ceiling or the top of the plastered area). Each face is measured separately, as internal and external plaster have different mortar mixes and thicknesses.
• Deductions (IS 1200 Part 12): openings up to 0.5 m² - no deduction; 0.5 to 3 m² - deduct one face only; above 3 m² - deduct both faces. Reveals are not added for openings up to 3 m², and are measured and added for larger openings.
• Plaster of skirting, dado, ceiling, band, and curved surfaces is measured separately where the rate differs. The thickness (12 or 15 mm), the mortar (1:4, 1:6) and the type of surface (brick, concrete) are stated in the item.
• Example: wall 5 m x 3 m with a door 1 m x 2.1 m on both plastered faces: area = 2 x (5 x 3) - 2.1 (once, as the opening is 0.5 to 3 m²) = 27.9 m².

ii) Cornice work

• Unit: running metre (m) for a cornice of given profile and girth. Where the cornice is large, it may be measured in m³ for masonry/concrete and m² for plaster.
• Method: the length is measured along the centre line of the cornice, over the wall faces on which it runs, without deduction for corners; the size (girth, projection, depth) and the mortar are stated in the description.
• Plastering of the cornice surface is measured along its girth, m x girth in m², unless included in the running metre item.

In a load bearing brick masonry building the walls carry the load and the foundation is a continuous strip below them. The quantities of earthwork, PCC, foundation masonry and superstructure are taken out for each wall by one of these methods.

1. Long wall - short wall method

• Choose the walls running in the longer direction as long walls, the others as short walls.
• Length of long wall (out-to-out) = centre-to-centre length + half the foundation (or wall) width at each end, $= L_c + b$.
• Length of short wall (in-to-in) = centre-to-centre length - half the width at each end, $= L_c - b$.
• Quantity = length x breadth x depth for each wall at every level (trench, PCC, footing, plinth, wall), because the widths change with depth.
• Suitable for: buildings with several rooms, walls of different thicknesses, and where the foundation has different widths for different walls.

2. Centre line method

• Find the total centre-line length of all walls (main and partition).
• Deduct for the junctions: for each T-junction/cross-junction deduct half of the wall width.
• Quantity = corrected total length x width x depth of each section.
• Suitable for: buildings having walls of the same section throughout, rectangular rooms, quick estimates. It is not accurate where the sections differ.

Procedure for both

1. Mark the walls on the plan with their centre lines.
2. Calculate the length of each as per the chosen method.
3. Take out earthwork, PCC, masonry up to plinth, DPC, then superstructure masonry.
4. Deduct for the openings (doors, windows, ventilators), lintels, and beams, to get net masonry.

In Nepal both methods are accepted; the centre line is quicker, and long wall-short wall is preferred for exactness.

The main factors considered in a detailed estimate are:

1. Completeness and accuracy of the drawings and specifications.
2. Quantities of each item, measured by the standard rules of measurement and the correct units.
3. Rates of materials, labour and transport (lead), based on the District Rate and DUDBC norms.
4. Site conditions: soil type, water table, access, haulage distance and availability of water and labour.
5. Method of construction and equipment to be used.
6. Time of construction and season, which influence rates and productivity.
7. Wastage allowances, overhead, profit, contingency and taxes (VAT 13%).
8. Local rules, safety and environmental requirements.

A work breakdown structure (WBS) divides the whole quantity-calculation job into smaller, manageable work packages, each with a clear owner and output. For an RCC frame building I would plan it as follows.

Step 1: Understand the scope

Collect the architectural and structural drawings (plans, sections, column layout, beam and slab details, schedules), the specifications and the BOQ format.

Step 2: Break down into levels

Level 0  Frame building project
 |
 +-- 1 Substructure
 |    1.1 Earthwork   1.2 PCC/soling
 |    1.3 Footings    1.4 Columns to plinth
 |    1.5 Plinth beam 1.6 Backfill, DPC
 +-- 2 Superstructure (per floor)
 |    2.1 Columns  2.2 Beams  2.3 Slab
 |    2.4 Stair    2.5 Brick/block walls
 |    2.6 Lintel, sunshade, parapet
 +-- 3 Finishing
 |    plaster, floor, paint, doors, windows
 +-- 4 Services (water, sanitary, electrical)
 +-- 5 External works (boundary, drain, road)

Step 3: Define each work package

For every package: concrete, formwork and reinforcement are taken separately (m³, m², kg), with the drawing reference, the formula, and the check procedure.

Step 4: Assign and schedule

Distribute the packages among the team members, set deadlines in the order of construction, and use a common quantity sheet format.

Step 5: Check and compile

Cross-check totals (for example concrete from columns + beams + slabs against the structural volume), consolidate into the BOQ, and prepare the abstract of cost.

The following points are considered when measuring earthwork:

1. Classification of soil: ordinary soil, soft rock, hard rock, and water-bearing strata are measured separately because their rates differ.
2. Dimensions: length x breadth x depth of the trench or pit, measured as the net volume in place (m³). The width of the foundation plus the working space (at least 0.15 to 0.3 m each side) and the depth to the bottom of the PCC.
3. Depth range: excavation is classified by depth (0 to 1.5 m, 1.5 to 3 m, and so on), and the lift and lead are specified.
4. Slopes and benching: side slopes in deep trenches, and benching, are added if allowed in the specification, otherwise the vertical sides are taken with shoring.
5. Lead and lift: the distance and the height of carrying the excavated soil away or to the filling site.
6. Dewatering, shoring and strutting when needed, measured as separate items.
7. Backfilling and disposal: the quantity of backfill = excavated volume - volume of foundation work below ground level. The surplus soil is disposed of or used for filling plinth.
8. Bulking and shrinkage: soil swells (about 20 to 30%) on excavation and compacts on filling.
9. No deduction for small items such as pipes, and no addition for the loosening of the soil.
10. Units: m³ for excavation/filling, m² for surface dressing, m for trimming along a line (as per the specification).

Components of a complete estimate

A complete estimate gives the total final cost of the project, including all costs related to the building besides the main construction cost:

1. Land cost and compensation, including legal charges.
2. Cost of the main building (civil works), from the detailed estimate.
3. Water supply, sanitary and drainage works.
4. Electrical installation (wiring, fittings, earthing, lightning conductor, lift).
5. Architectural treatment and special finishing.
6. External works: approach road, compound wall, gate, landscaping, septic tank.
7. Furniture, equipment and fittings.
8. Planning, design and supervision charges (consultant fees), and establishment.
9. Contingency (3 to 5%) and work-charged establishment.
10. Taxes (VAT 13%), interest during construction, and price escalation provisions.

Sample abstract of cost

(Illustrative quantities and rates)

Rates are illustrative; actual rates are taken from the District Rate.

Assumed: a single-room residential building, clear size 4 m x 3 m, 230 mm brick walls, 3.0 m high, one door 1.0 x 2.1 m and one window 1.2 x 1.2 m; trench 0.9 m wide x 1.0 m deep; centre line length $L = 2(4.23 + 3.23) = 14.92$ m.

Quantity sheet (detailed measurement)

Abstract of cost (rates illustrative)

Contingency, VAT and the other items (RCC slab, roofing, doors, painting) would be added in the same way.

Abstract of cost (AOC)

Amount = quantity x unit rate.

Bill of quantities (BOQ)

(The rate and amount columns of the BOQ are filled by the bidder; with the rates above it becomes the priced BOQ shown in the abstract.)

Summary of cost

The 13% VAT is applied on the cost including contingency, as is the usual Nepal practice.

Answer: Total of works = Rs 937,500.00; with 5% contingency = Rs 984,375.00; grand total with 13% VAT = Rs 1,112,343.75.

A bill of quantities lists each item with its description, unit, quantity, rate and amount (amount = quantity x rate).

Working of the amounts:

• PCC for RCC: 108 x 13,200 = 1,425,600.00
• Formwork: 3,240 x 750 = 2,430,000.00
• Reinforcement: 21,600 x 115 = 2,484,000.00
• Wearing coat: 18 x 12,090 = 217,620.00

Answer: Total cost of the T-beam decking works = Rs 6,557,220.00. Contingency and VAT are added to this total if required.

Quantities are measured net, so openings and voids are deducted as per IS 1200 and the Nepal practice. The rules depend on the size of the opening.

Brickwork (IS 1200 Part 3)

• Deduct all openings (doors, windows, ventilators) in full volume: opening area x wall thickness.
• Openings up to 0.1 m²: no deduction. Bearing of lintels and beams that are of concrete are deducted; the end of joists, rafters and so on: no deduction.
• Reinforced concrete lintels, beams, columns inside the wall are deducted from brickwork.

Plastering (IS 1200 Part 12)

Painting (IS 1200 Part 15)

• Painting on plastered walls follows the plaster rule; openings up to 0.5 m² are not deducted.
• For doors and windows, painting is measured on the opening area with the multiplying factors of IS 1200 (for example 1.30 for panelled, 1.80 for louvered).

Example

Brick wall 6 m x 3 m x 0.23 m thick, with a window 1.2 x 1.2 m (1.44 m²) and a door 1.0 x 2.1 m (2.10 m²), plastered on both faces.

• Brickwork: 6 x 3 x 0.23 = 4.14 m³; deduct (1.44 + 2.10) x 0.23 = 0.814 m³; net = 3.33 m³.
• Plaster (both faces): gross = 2 x 18 = 36 m². Both openings are between 0.5 and 3 m², so deduct one face only: 36 - 3.54 = 32.46 m².
• Painting of the plastered surface: same as the plaster quantity, 32.46 m².

Painting of doors and windows is measured as the area of the opening (length x breadth, including the frame), taken on one face. As the surface is much more than the plain area (panels, mouldings, louvers, edges, frame, etc.), the area is multiplied by a factor that accounts for the extra surface. Typical factors as per IS 1200 (Part 15), for each side painted:

Reasons: a panelled door has recessed panels and mouldings, so surface area exceeds the plane area by 30%. A flush door has both flat faces plus edges, so the factor is slightly above 1. A louvered door has slats at an angle with both sides exposed, so the factor is large. A glazed one has part of the opening covered by glass, so less surface needs painting, but the frame, bars and edges still do.

Example: panelled door 1.0 x 2.1 m = 2.1 m²; painting area per side = 2.1 x 1.30 = 2.73 m². The factors should be checked against the current edition of the IS code or the Nepal norms used.

Doors are painted over a larger surface than the plain opening area because of panels, mouldings and slats. To simplify measurement, the area of the door opening (including the frame) is multiplied by a factor.

Use: painting quantity = width x height of the door opening x factor (for each painted side). For example, a panelled door of 1.0 x 2.1 m painted on one side = 2.1 x 1.30 = 2.73 m², and a louvered door of the same size = 2.1 x 1.80 = 3.78 m².

The centre line method is a quick way of finding the quantities of earthwork, foundation and wall in a building that has walls of the same section.

Procedure

1. Find the total centre-line length of all the walls (main and partitions), measured along the middle of the walls.
2. Deduct for the junctions where walls meet: each T-junction takes away half of the width of the wall (the part counted twice); a cross-junction deducts the same on each side.
3. Multiply the corrected length by the width and the depth of each section (trench, PCC, footing, plinth masonry, superstructure).

Formula

$$L_{corrected} = L_{total} - n \times \frac{b}{2}$$

where $n$ is the number of junctions and $b$ the width of the wall at that level.

Example

For a room 4 m x 3 m (clear) with a 0.23 m wall: $L = 2(4.23 + 3.23) = 14.92$ m. A partition with two T-junctions would deduct $2 \times 0.23/2 = 0.23$ m. The trench volume = $14.92 \times 0.9 \times 1.0 = 13.43$ m³.

Merits and limits

It is fast and simple, and suited to uniform rectangular buildings; it is not accurate where the wall sections differ, for which the long wall-short wall method is used.

A semicircular arch has a rise equal to half the span. The brickwork quantity of the arch is the volume of the arch ring (the area between the intrados and the extrados) multiplied by the wall thickness.

        extrados  R = r + t
      .-----------.
    .' .---------. '.
   /  /  intrados \  \
  |  |     r      |  |
  |__|____________|__|
     <---- span --->
        S = 2r

Method

1. Span $S = 2r$, so the radius of the intrados is $r = S/2$ and the rise equals $r$.
2. Radius of the extrados: $R = r + t$, where $t$ is the thickness (depth) of the arch ring.
3. Area of the arch ring (half annulus):

$$A = \frac{\pi}{2}\left(R^2 - r^2\right)$$

4. Quantity of arch masonry: $V = A \times b$, where $b$ is the thickness of the wall (the width of the arch along the wall).
5. Brickwork above the opening (solid wall between the arch and the lintel level) is measured separately; the opening below the springing line is deducted from the wall as a rectangle $S \times$ height up to springing, and the semicircle $\frac{\pi r^2}{2} \times b$ is deducted above it, and then the arch ring is added at the arch masonry rate.

Worked example (assumed values)

Span 2.0 m ($r = 1.0$ m), arch thickness $t = 0.30$ m ($R = 1.30$ m), wall thickness $b = 0.23$ m.

$$A = \frac{\pi}{2}(1.30^2 - 1.00^2) = 1.0838\ \text{m}^2, \qquad V = 1.0838 \times 0.23 = 0.249\ \text{m}^3$$

Opening to be deducted from the wall (semicircle above the springing line): $\frac{\pi}{2}(1.0)^2 \times 0.23 = 0.361$ m³.

Answer: Arch ring masonry = 0.249 m³ (for the assumed dimensions).