Home & Construction⏱ 4 min read
How to Calculate Concrete Mix Ratios by Weight
Volume ratios are how concrete is traditionally specified but weighing gives more consistent results. Here is how to convert between the two, calculate batch quantities, and adjust for aggregate moisture.
Most DIY concrete guides give ratios by volume (1:2:3). Professional concrete is batched by weight because aggregate varies in density and moisture content. Understanding both systems helps you get stronger, more consistent results.
Standard Mix Ratios and Their Uses
Mix Ratio (C:S:A)Strength ClassW/C RatioUse
1:3:6C100.75-0.80Blinding, fill
1:2:4C200.60-0.65Footings, paths
1:1.5:3C250.50-0.55Slabs, drives
1:1:2C350.40-0.45Structural beams
C:S:A = Cement : Fine aggregate (sand) : Coarse aggregate (gravel/stone)
Converting Volume to Weight Ratios
Bulk densities (approximate):
Cement: 1,300 kg/m3
Dry sand: 1,600 kg/m3
Gravel (20mm): 1,700 kg/m3
Water: 1,000 kg/m3
1:2:4 mix by volume, convert to weight:
Cement: 1 part x 1,300 = 1,300 kg (reference unit)
Sand: 2 parts x 1,600 = 3,200 kg
Gravel: 4 parts x 1,700 = 6,800 kg
Weight ratio: 1,300 : 3,200 : 6,800
Simplified: 1 : 2.46 : 5.23 (by weight)
Water-cement ratio (W/C = 0.60):
Water = 0.60 x cement weight = 0.60 x 1,300 = 780 kg per m3 cement
Batch Calculation for a Small Pour
Pour: garden path slab, 3m x 1.2m x 100mm thick
Volume needed: 3 x 1.2 x 0.1 = 0.36 m3
Add 10% for waste: 0.36 x 1.1 = 0.396 m3
Using C20 mix (1:2:4 by volume):
Dry ingredients expand when water is added, then shrink when cured.
Approximate yield factor: 0.65 (dry mix volume to cured concrete volume)
Dry mix volume needed: 0.396 / 0.65 = 0.609 m3
Total parts by volume: 1+2+4 = 7
Cement: 0.609 x (1/7) = 0.087 m3 = 87 litres
At 1,300 kg/m3: 87 x 1.3 = 113 kg = 4.5 x 25kg bags
Sand: 0.609 x (2/7) = 0.174 m3 = 174 litres = ~278 kg
Gravel: 0.609 x (4/7) = 0.348 m3 = 348 litres = ~592 kg
Moisture Adjustment for Aggregate
Sand absorbs and holds moisture. Wet sand contains free water
that must be deducted from the mix water.
Moisture content test:
Weigh sample (e.g. 500g), dry in oven to constant weight, reweigh.
Moisture content % = (wet weight - dry weight) / dry weight x 100
Typical site sand moisture: 3-7%
At 5% moisture on 278kg sand:
Free water in sand: 278 x 0.05 = 13.9 kg
Reduce mix water by 13.9 litres
Original water needed: 113 x 0.60 = 67.8 litres
Adjusted water: 67.8 - 13.9 = 53.9 litres
This adjustment explains why on-site slumps vary with weather:
Wet weather = wet sand = need less mix water = stiffer apparent mix
Many DIYers add more water to compensate, weakening the concrete.