Theory: Fineness Modulus (FM)

Fineness Modulus (FM) is an empirical index that represents the average size of particles in a fine aggregate. It is obtained from sieve analysis and provides a single numerical value to assess whether sand is fine, medium, or coarse.

FM does not represent any specific sieve size; instead, it is a grading index useful for comparing aggregates and controlling consistency of concrete mixes.

🔹 Standard Sieve Series Used

As per Indian practice, FM of fine aggregate is calculated using the following sieves:

• 4.75 mm (No. 4)
• 2.36 mm (No. 8)
• 1.18 mm (No. 16)
• 0.600 mm (No. 30)
• 0.300 mm (No. 50)
• 0.150 mm (No. 100)

Material passing 0.150 mm (pan) is excluded from FM calculation.

🔹 Formula for Fineness Modulus

FM = (Sum of cumulative % retained on standard sieves) ÷ 100

🔹 Significance of Fineness Modulus

• Controls workability of concrete
• Influences cement and water demand
• Helps maintain uniformity of sand supply
• Used in mix proportioning as per IS method

🔹 Typical FM Ranges for Fine Aggregate

FM Range	Sand Classification	Engineering Use
< 2.2	Very Fine	Plaster, high water demand
2.2 – 2.6	Fine	Mortar, controlled concrete
2.6 – 3.0	Medium	Most RCC works (preferred)
> 3.0	Coarse	Lean concrete, high strength mixes

🔹 Relevant Indian Standards

• IS 2386 (Part 1): Methods of test for aggregates – Sieve analysis
• IS 383: Specification for coarse and fine aggregates
• IS 456: Plain and reinforced concrete – Code of practice
• IS 10262: Concrete mix proportioning – Guidelines

Note: Fineness Modulus should always be used together with grading curves, zone classification, and silt content to ensure durable and workable concrete.

Fineness Modulus (FM) Calculator

Enter weight retained on each sieve (in grams). Default sample weight = 1000 g.

Sample weight (g)

Auto-normalize to sample?
No (use entered weights) Yes (scale weights so sum = sample weight)

Sieve (mm)	Weight retained (g)	% Retained	Cumulative % Retained

Sum of cumulative % (standard sieves excluding pan): 0

Fineness Modulus (FM) = Sum / 100 = 0.00

FM is calculated using cumulative % retained on the standard sieves (4.75, 2.36, 1.18, 0.600, 0.300, 0.150 mm). Pan is excluded from the FM sum.