---
title: "Fineness modulus of aggregate"
subject: "Concrete Technology and Masonry Structure"
chapter: 1
key: "semester_5/concrete_technology_and_masonry_structure:fineness-modulus-aggregate"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/fineness-modulus-aggregate
site: BCT Engineering
---

# Fineness modulus of aggregate

Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials

- Asked in: 2076 Chaitra, 2 marks; 2067 Ashadh (old course), 5 marks (asked 2 times)
- Video: [How To Calculate Fineness Modulus Of Aggregate? | FM Of Aggregate Numerical | Civil Concept](https://www.youtube.com/watch?v=zmEA9Di-0oI) — Civil Concept
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/fineness-modulus-aggregate
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1

## Question

Define fineness modulus of aggregate. How does the fineness modulus of aggregate impact concrete? Describe with suitable examples.

## Short answer

Fineness modulus is the sum of cumulative percentages retained on sieves 80 mm to150 µm divided by100. A higher FM indicates coarser aggregate, which reduces water and cement demand but can cause harshness; a lower FM increases water demand and paste. It is used to adjust mix proportions and check aggregate uniformity.

## Answer

**Fineness modulus (FM)** is an empirical number obtained by adding the cumulative percentages retained on the standard sieves (80, 40, 20, 10, 4.75, 2.36 and 1.18 mm, and 600, 300 and 150 micron) and dividing by 100. It indicates the mean size of the particles in the aggregate.

$$
FM = \frac{\sum \text{cumulative percentage retained}}{100}
$$

The 150 micron sieve is the last one included. A larger FM means a coarser aggregate. Typical values: fine aggregate 2.0-3.5 (commonly classed as fine sand 2.2-2.6, medium sand 2.6-2.9 and coarse sand 2.9-3.2), coarse aggregate 5.5-8.0, and all-in aggregate 3.5-6.5.

#### Impact on concrete
- A higher FM (coarser) means less surface area, so less water and cement paste for a given workability; but too coarse a sand gives harsh, segregation-prone mixes.
- A low FM (fine sand) increases water demand, needs more cement and reduces strength, but gives a cohesive mix.
- In mix design, the FM of the fine aggregate is used to select the coarse aggregate volume (ACI method): sand with a higher FM needs a smaller amount of coarse aggregate.
- FM is used to check the uniformity of supply: a change in FM of more than 0.2 indicates a change in grading and needs an adjustment of the mix.

#### Example
For a sand, the percentages retained on the 4.75 mm, 2.36 mm, 1.18 mm, 600, 300 and 150 micron sieves are 5, 10, 15, 25, 30 and 12 (cumulative 5, 15, 30, 55, 85, 97). The sum of the cumulative values = 287, so FM = 2.87, which is a medium sand (Zone II). In the ACI table, with 20 mm aggregate this sand (FM 2.87) takes a dry-rodded coarse aggregate volume of about 0.61 per m$^3$ of concrete. A finer sand (FM 2.4) allows more coarse aggregate (0.66), and a coarser sand (FM 3.0) allows less (0.60).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Fineness modulus is:
   A. Sum of cumulative percentages retained / 100
   B. Sum of percentage passing / 100
   C. Average of sieve sizes
   D. Ratio of sand to cement
   Answer: A. It is an index of mean particle size. (Key point: Definition)

2. A larger FM indicates:
   A. Finer aggregate
   B. Coarser aggregate
   C. More silt
   D. Higher moisture
   Answer: B. Larger FM means coarser. (Key point: Meaning)

3. Cumulative retained 5,15,30,55,85,97 gives FM:
   A. 28.7
   B. 3.87
   C. 2.87
   D. 1.87
   Answer: C. Sum is 287, divided by 100. (Key point: Calculation)

4. A low FM sand:
   A. Reduces water demand
   B. Is harsh and segregates
   C. Allows more coarse aggregate
   D. Raises water demand, needs more cement, but is cohesive
   Answer: D. Fine sand has large surface area. (Key point: Effect)

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