How this calculator works
Concrete mix ratios are quoted by volume of dry ingredients: 1 part cement, 2 parts sand, 3 parts gravel for the standard mix. But the dry volume of those ingredients is bigger than the finished wet concrete by about 54% — that's the standard 1.54 dry-volume factor, which accounts for the voids in the dry materials that water and cement paste fill in during mixing.
The calculator applies that factor automatically. Type the wet concrete volume you want; the calculator scales up for the dry ingredients you'll need to buy.
Cement is converted to bags using a density of 94 lb per cubic foot (the standard for Portland cement) and your chosen bag size. Water uses a 0.5 water-cement ratio by weight, which is the standard for general-purpose concrete (lower W/C = stronger, higher = more workable).
Mix ratios at a glance
| Ratio (C:S:G) | Strength (psi) | Best for | Cement per yd³ |
|---|---|---|---|
| 1 : 1.5 : 3 | ~4,000 | Beams, structural columns | ~7–8 bags |
| 1 : 2 : 3 | ~3,000 | General DIY: slabs, footings, walks | ~6 bags |
| 1 : 2 : 4 | ~2,500 | Non-structural slabs, post setting | ~5 bags |
| 1 : 3 : 5 | ~1,800 | Mass concrete, fills | ~3.5 bags |
Materials for one cubic metre (1:2:3 mix)
Buying by the cubic metre? For a standard 1:2:3 mix, one m³ of finished concrete needs roughly the quantities below. Cement is shown in 25 kg bags (UK/EU) and 94 lb sacks (US). The 1.54 dry-volume factor is baked in, so these are what you actually buy, not the settled wet volume.
| Ingredient | Per m³ of concrete | In bags |
|---|---|---|
| Cement | ~370 kg | ~15 × 25 kg (or ~9 × 94 lb) |
| Sand | ~0.5 m³ (~0.8 t) | — |
| Gravel | ~0.77 m³ (~1.2 t) | — |
| Water | ~185 litres (~49 gal) | — |
A leaner 1:2:4 mix drops the cement to about 300 kg (~12 × 25 kg bags) per m³ and bumps the gravel, trading strength for cost. For a batch smaller than a cubic metre, scale everything down by the same fraction — the calculator does this automatically once you enter your volume.
How to use this calculator
- Find your concrete volume. A 4-inch slab over 100 sq ft is 33 cuft (or 1.23 yd³). Multiply length × width × thickness (all in feet) to get cubic feet of finished concrete.
- Pick a mix ratio. 1:2:3 (cement:sand:gravel by volume) for general DIY work — slabs, posts, footings, walks. 1:2:4 for non-structural fills. 1:3:5 for mass concrete and lower-strength applications.
- Choose your cement bag size. 94 lb US standard sack, 25 kg UK/EU bag, or 40 kg commercial bag. The calculator rounds bag count up.
- Read off the quantities. Cement (in bags), sand (in yd³ or cuft), gravel (in yd³ or cuft), and water (in gallons). The 1.54 dry-volume factor is already applied to account for void filling during mixing.
- Buy 10–15% extra. Concrete mixing wastes a small amount, and bagged cement comes only in whole bags. Round up.
Worked example: 4" slab over 100 sq ft
A common DIY pour: 10 ft × 10 ft × 4 in = 33.3 cuft of finished concrete.
- Dry-volume scale-up: 33.3 × 1.54 = 51.3 cuft of dry ingredients
- Cement (1/6 of dry volume): 8.55 cuft × 94 lb/cuft = 803 lb → 9 bags of 94 lb cement
- Sand (2/6 of dry volume): 17.1 cuft = 0.63 yd³
- Gravel (3/6 of dry volume): 25.6 cuft = 0.95 yd³
- Water (0.5 W/C): 803 × 0.5 ÷ 8.345 = 48 gallons
At rough US prices: 9 bags × $14 = $126 in cement, ~$15 in concrete sand, ~$30 in gravel, and ~$5 in water. Total under $200 in raw materials for the slab.