WORKED MIX DESIGN EXAMPLE

M20 concrete ratio and mix design

Work through an M20 mix design: cement, sand, aggregate and water for 1 m³, the durability minimum and a scalable batch worksheet.

M20 · Mild exposureReinforced concrete · OPCIllustrative materials · Not a tested mix

Scale the worked example

Required concrete volume · m³

This M20 example shows why selecting a grade does not produce one fixed cement:sand:stone recipe. In this brief, the minimum cement content slightly increases the binder above the strength-ratio estimate.

All material assumptions are listed below. Open the full lab to change them.

M25 example →M30 example →

YOUR M20 EXAMPLE · BATCH QUANTITIES

2,447.56kg for 1 m³ designed concrete

Target mean strength26.6 MPa

Final water / cement0.5258

SSD ratio by mass1 : 2.5 : 4.12

Ratio is total binder : fine aggregate : coarse aggregate for this example. Batch weights below use the entered aggregate moisture.

Material	Your batch · kg
Cement	300
Fine aggregate (as batched)	749.58
Coarse aggregate (as batched)	1,237.25
Water to add at mixer	157.73
Chemical admixture	3

Adjust this M20 design in the lab →

When minimum cement content controls the mix

The reference ratio and final ratio can differ. Here, the minimum content for the selected reinforced-concrete exposure governs the binder amount. The free-water budget stays the same, so the final water/cement ratio falls. Adding more water to restore the earlier ratio would change the design.

M20 is the minimum grade shown by the selected IS 456 table check for mild reinforced-concrete exposure. If the project is in moderate exposure, that same check requires M25. Change the exposure in the full calculator instead of using this mild-exposure example unchanged.

Material and design assumptions

Slump / aggregate: 100 mm / 20 mm angular, Zone II
Specific gravities: Cement 3.15; sand 2.65; stone 2.74
Entrapped air: 1% of concrete volume
Water reduction / dosage: 20% trial assumption / 1% of binder
Sand: absorption / moisture: 1% / 1% of oven-dry mass
Stone: absorption / moisture: 0.5% / 0.5% of oven-dry mass
Cement-strength curve: Expected cement strength 43 to <53 MPa; approximate reference
Admixture SG / water fraction: 1.145 / 0% — replace with supplier data
Binder / statistical basis: OPC only; assumed standard deviation; good control

These are example inputs, not a material test certificate. No fly ash, GGBS, silica fume, pump reduction or binder uplift is applied. The reference ratio is selected from the approximate curve, rounded down to 0.01 and checked against the selected durability cap.

Is M20 always a 1:1.5:3 concrete mix?

The familiar bucket shorthand is not the output of a material-specific design. This worksheet reports SSD proportions by mass, with measured properties and moisture needed for batching. Nominal proportioning and designed mixes are different procedures; a label alone does not prove strength.

Follow the M20 calculations

1. Set the target mean strength

IS 10262:2019 · 4.2, Tables 1–2

f′ck = max(fck + 1.65S, fck + X)max(20 + 1.65 × 4.00, 20 + 5.50) = 26.60 MPa

Assumed standard deviation 4.00 MPa. The larger margin governs; characteristic and target strengths are different.

2. Select and check the water/binder ratio

IS 10262 · 5.1 / IS 456 · Table 5

r = min(strength-based ratio, durability limit)min(0.53, 0.55) = 0.53

The ratio is interpolated from approximate Figure 1 readings and rounded down to 0.01. It is a preliminary selection, not a measured strength prediction.

3. Estimate the free water

IS 10262 · 5.3, Table 4

W = (W₀ − shape adjustment) × slump factor × (1 − reduction)(186 − 0) × 1.060 × 0.80 = 157.7 kg/m³

The starting water estimate is for SSD aggregates. The 3% change per 25 mm slump and admixture reduction are preliminary; actual workability must be measured.

4. Calculate and allocate the binder

IS 10262 · 5.4 / IS 456 · Tables 5–6

B = max((W ÷ r) × trial increase, durability minimum)max((157.73 ÷ 0.53) × 1.00, 300.00) = 300.0 kg/m³

Final w/b is 0.526 after minimum-content or trial adjustments. Replacement percentages are fractions of total binder mass.

5. Close the absolute-volume balance

IS 10262 · 5.2, 5.7

Vagg = 1 − Vair − Σ [mᵢ ÷ (Gᵢ × 1000)]1 − 0.0100 − 0.2556 = 0.7344 m³

Use specific gravity, not loose bulk density. Water carried inside the chemical admixture is counted once, within that product’s volume.

6. Split the aggregate volume

IS 10262 · 5.5, Table 5

pCA = [p₀ + 0.2(0.50 − w/b)] × (1 − reduction)[0.62 + 0.2 × (0.50 − 0.526)] × 1.00 = 61.5% coarse / 38.5% fine

SSD masses: fine 749.6 kg and coarse 1237.3 kg per m³. The optional reduction is a trial adjustment up to 10%, not an automatic pumpability guarantee.

7. Correct for aggregate moisture

SSD mass balance · IS 10262 Annexes A–B

mOD = mSSD/(1+A); mwet = mOD(1+MC); Wsurface = mOD(MC−A); Wmixer = Wfree − Wsurface,sand − Wsurface,stone − Wadmixture

Sand surface water: 742.2 × (1−1)/100 = 0.00; stone surface water: 1231.1 × (0.5−0.5)/100 = 0.00; mixer water: 157.73 − (0.00) − (0.00) − 0.00 = 157.7 kg/m³ mixer water

All water quantities above are kg per m³. A and MC are fractions in the formula; entered percentages are divided by 100. Total moisture and absorption use an oven-dry mass basis. Surface water 0.0 kg plus admixture water 0.0 kg is deducted from the free-water budget. Negative surface water represents aggregate absorption demand.

8. Scale and test the trial batch

IS 10262 · 5.8

Batch quantity = quantity per m³ × batch volumeEach constituent × 1.000 m³ = 1.000 m³ trial batch

Measure workability, fresh density, yield and strength; inspect segregation and bleeding. Revise the mix and record trials before project approval.

Checks and assumptions to review

Admixture water contribution is set to 0%. Enter the supplier’s measured water fraction when applicable; do not assume a liquid product is water-free.

Method: IS 10262:2019; selected durability provisions: IS 456:2000. BIS-authored third-party source copies. Verify applicable editions, amendments and project requirements. No independent professional approval is claimed.