Show the math
- Convert to base units: Finished (wet) concrete needed 1 m³ = 35.3147 ft³
- Wet volume with 10% waste = 1.1 m³
- Dry volume = wet × 1.54 = 1.694 m³
- Parts = 1 + 2 + 4 = 7
- Cement = 1.694 × 1 ÷ 7 = 0.242 m³ → 348 kg at 1,440 kg/m³
- Cement bags = 348 ÷ 50 kg = 6.97 → 7
How to measure
Enter the wet volume of finished concrete in cubic meters, cubic yards or cubic feet. The ratio is by volume, written cement:sand:gravel, such as 1:2:4. The calculator adds your waste percentage, multiplies by the dry-volume factor, and divides the dry volume in proportion to the ratio.
Ratios by volume are approximate. Moist sand bulks, the aggregates differ in density, and the water content changes the final volume. For structural work use a designed mix, not a ratio.
Why dry materials need more volume
When you mix cement, sand and gravel with water, the cement paste fills the gaps between the sand grains, and the sand and paste fill the gaps between the gravel. The result takes up less volume than the dry materials had on their own. The ratio between dry volume and finished volume is called the dry-volume factor. A figure of about 1.54 is commonly used, with values between 1.52 and 1.57 seen in practice. The calculator uses 1.54 and lets you change it.
Formula: dry volume = wet volume × factor. Cement = dry × c ÷ (c + s + g). Bags = cement kg ÷ 50 kg.
Worked example: 1 m³ of 1:2:4 concrete
- Convert to base units: Finished (wet) concrete needed 1 m³ = 35.3147 ft³
- Wet volume with 10% waste = 1.1 m³
- Dry volume = wet × 1.54 = 1.694 m³
- Parts = 1 + 2 + 4 = 7
- Cement = 1.694 × 1 ÷ 7 = 0.242 m³ → 348 kg at 1,440 kg/m³
- Cement bags = 348 ÷ 50 kg = 6.97 → 7
For 1 m³ with 10% waste, you need 7 × 50 kg bags (348 kg for 1.10 m³ of concrete at 1:2:4). The sand and gravel quantities are shown in the result card in cubic meters and cubic feet. Cement is converted using 1440 kg per cubic meter of loose cement powder, and 50 kg bags.
Common volume ratios
| Ratio | Name | Typical use |
|---|---|---|
| 1:1.5:3 | Rich | Structural elements, reinforced beams and columns, where specified |
| 1:2:4 | Standard | General reinforced work, slabs, floors |
| 1:3:6 | Lean | Blinding, mass fill, non-structural base |
These are traditional rules of thumb. Modern structural concrete is specified by strength and durability, not by a ratio.
Water and strength
The amount of water has a larger effect on strength than most people expect. A mix with too much water is easier to place and weaker once it hardens. The right amount is stated by the supplier or by the water-cement ratio in the mix design, not by the volume ratio. For bagged mixes the manufacturer states the water per bag on the label. See the mix ratios guide.
Using the result
- Buy cement in bags of 50 kg (110 lb) and round up.
- Sand and gravel are sold by volume or by weight. Ask the supplier for the bulk density to convert.
- Moist sand takes up more space (bulking). Dry out or add a margin if the sand is wet.
- Always mix a trial batch and check workability before the full pour.
- Mix in a mechanical mixer for anything over a few barrows.
Mistakes to avoid
- Using the dry volume as the finished volume. The result will be 35% short.
- Using shovelfuls as “parts” unless the shovels are the same size.
- Adding water until the mix looks easy to pour.
- Using a ratio for a structural pour without engineering input.
- Forgetting waste.
A small batch example
For half a cubic meter of 1:2:4 concrete with 10% waste you need 4 × 50 kg bags (174 kg for 0.55 m³ of concrete at 1:2:4). In practice, measure the parts with a bucket or a gauge box of known volume, mix a trial batch first and note the water that produces the right consistency. Keep the same measuring for every batch, and keep a record, so that batches are consistent from the first to the last.
Frequently asked questions
What is the 1:2:4 concrete mix?
One part cement, two parts sand and four parts gravel by volume. It is a traditional general-purpose mix.
How much cement is in a cubic meter of concrete?
For a 1:2:4 mix with the 1.54 factor, a little over 6 bags of 50 kg (about 317 kg) before waste. Designed mixes vary.
Why 1.54?
It is the commonly used ratio of dry volume to wet volume. Values from 1.52 to 1.57 are used in practice.
What is the water-cement ratio?
The weight of water divided by the weight of cement. Lower ratios give higher strength and better durability, if the concrete can still be placed.
Can I use this for bagged concrete?
No. Bagged mix already contains cement, sand and gravel in set proportions. Use the bag calculator.
Is the result exact?
No. It is an estimate that depends on moisture, bulking and aggregate shape. Do a trial batch first.
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.