How to Use This Calculator
- Enter your project's length, width, and thickness, or switch to direct volume entry if you already know your cubic yardage or cubic metreage.
- Choose a mix ratio from the preset list, or select Custom to enter your own cement to sand to aggregate proportions.
- Adjust the water-cement ratio if needed. The default of 0.5 works for most general-purpose mixes.
- Select your cement bag size so the calculator can tell you exactly how many bags to buy, not just a raw weight.
- Add a waste percentage. We default to 10% to cover spillage and mixing losses.
- Read your results: the exact weight of cement, sand, and aggregate needed, the number of cement bags, and how much water to add.
How We Calculate a Concrete Mix Ratio
A concrete mix ratio describes the proportions of cement, sand, and aggregate by volume. A 1:2:4 ratio means 1 part cement, 2 parts sand, and 4 parts coarse aggregate, for a total of 7 parts.
The tricky part is that dry materials take up more space than the finished wet concrete, since dry particles have air gaps between them that get filled in once water is added and everything compacts together. To account for this, the industry standard is to multiply your wet concrete volume by 1.54 to get the dry material volume you actually need to buy.
Formula: Dry Volume = Wet Volume x 1.54; Total Ratio Parts = Cement Parts + Sand Parts + Aggregate Parts; Cement Volume = (Cement Parts / Total Ratio Parts) x Dry Volume; Sand Volume = (Sand Parts / Total Ratio Parts) x Dry Volume; Aggregate Volume = (Aggregate Parts / Total Ratio Parts) x Dry Volume
Each material's volume is then converted to weight using its typical bulk density: cement at about 1,440 kg per cubic metre (90 lbs per cubic foot), sand at about 1,600 kg per cubic metre (100 lbs per cubic foot), and aggregate at about 1,450 to 1,550 kg per cubic metre (90 to 97 lbs per cubic foot).
Worked example: A 1 cubic metre pour using a 1:2:4 ratio works out as follows. Dry volume = 1 x 1.54 = 1.54 cubic metres. Total ratio parts = 1 + 2 + 4 = 7. Cement volume = (1/7) x 1.54 = 0.22 cubic metres, which is about 317 kg of cement, or roughly 13 bags at 25 kg each. Sand volume = (2/7) x 1.54 = 0.44 cubic metres, about 704 kg. Aggregate volume = (4/7) x 1.54 = 0.88 cubic metres, about 1,320 kg.
Common Concrete Mix Ratios
| Ratio Cement:Sand:Aggregate | Approx. Grade US/UK | Typical Use |
|---|---|---|
| 1:4:8 | M7.5 | Mass fill, leveling, non-structural work |
| 1:3:6 | M10 | Mass concrete, blinding layers, non-structural fill |
| 1:2:4 | M15 / approx. C20 | Shed bases, garden paths, patios, footings, general-purpose domestic work |
| 1:2:3 | approx. C25 | Driveways, garage floors, anywhere vehicles park or drive |
| 1:1.5:3 | M20 / approx. C25 to C30 | Structural slabs, beams, and columns in low-rise residential construction |
| 1:1:2 | M25 | Higher-strength structural elements |
A 1:2:4 mix is the most commonly used ratio for general domestic work, since it balances adequate strength with cost and is easy to mix by hand or with a small mixer. For anything structural, load-bearing, or above a couple of stories, always confirm the required mix with a structural engineer or your local building code rather than relying on a nominal mix alone.
Water-Cement Ratio
The water-cement ratio controls how workable your mix is and how strong it ends up once cured. A lower ratio produces stronger concrete, but a mix that's too dry is difficult to place and compact properly.
- 0.4 to 0.45: Higher strength, less workable, typically needs a plasticizer additive to place easily
- 0.5: A good general-purpose default for most domestic mixes
- 0.55 to 0.6: More workable, easier to place by hand, but noticeably lower final strength
As a rule, add water gradually rather than all at once, and stop as soon as the mix holds its shape without being soupy. Adding too much water is one of the most common causes of weak, cracked concrete, since excess water that doesn't react with the cement simply leaves voids behind as it evaporates.
Converting Cement to Bags
Cement is sold in standard bag sizes that vary by region, so the calculator converts your calculated cement weight into a bag count automatically.
| Bag Size | Region | Approx. Volume per Bag |
|---|---|---|
| 94 lb bag | US (standard cement bag, not premixed concrete) | About 1 cubic foot |
| 50 kg bag | UK / most other regions | About 0.035 cubic metres |
| 25 kg bag | UK (smaller, common for DIY) | About 0.0167 cubic metres |
Note that this is different from the premixed 40, 60, and 80 lb bags covered on our main Concrete Calculator. Those are pre-blended bags that already contain cement, sand, and aggregate together, ready to mix with water alone. This page is specifically for site-mixing your own concrete from separate raw materials using a chosen ratio.
Tips for Mixing Your Own Concrete
- Mix the dry materials first. Combine cement, sand, and aggregate thoroughly until the color is uniform before adding any water.
- Add water gradually, in two or three stages, checking the mix's workability after each addition rather than pouring it all in at once.
- Use clean, sharp sand. Sand contaminated with clay, silt, or salt weakens the finished concrete significantly.
- Match aggregate size to your pour. Smaller aggregate suits thinner sections; larger aggregate is better for mass pours over 12 inches thick.
- Order 5 to 10% extra material on top of your calculated quantities to account for spillage, mixing losses, and minor variations in your forms or ground level.
Frequently Asked Questions
What does a 1:2:4 concrete mix ratio mean?
It means 1 part cement, 2 parts sand, and 4 parts coarse aggregate, by volume. It's one of the most common general-purpose mixes, roughly equivalent to M15 grade or C20 in the UK, suitable for shed bases, patios, and footings.
How much cement, sand, and aggregate do I need for 1 cubic metre of concrete?
For a standard 1:2:4 mix, 1 cubic metre of finished concrete needs about 317 kg of cement, 634 kg of sand, and 1,268 kg of aggregate, once the 1.54 dry volume factor is applied.
What is the best concrete mix ratio for a driveway?
A richer mix than standard patio or shed work, typically 1:2:3 or 1:1.5:3, since driveways need to support vehicle weight. Confirm the right grade for your specific vehicle loads with a supplier or engineer.
What's the difference between this calculator and your main concrete calculator?
This calculator is for mixing your own concrete from raw cement, sand, and aggregate using a chosen ratio. Our main Concrete Calculator is for ordering ready-mix or premixed bags by volume, without needing to think about individual material ratios.
How many bags of cement do I need for a 1:2:4 mix?
It depends on your total volume. As a reference, 1 cubic metre of 1:2:4 concrete needs roughly 317 kg of cement, which is about 13 bags at 25 kg each, or about 6 bags at 50 kg each.
Why do I need to multiply by 1.54 when calculating dry materials?
Dry cement, sand, and aggregate have air gaps between the particles that get filled in once water is added and the mix compacts. The 1.54 factor accounts for that difference, so you buy enough dry material to actually produce your target volume of finished concrete.
What is ballast, and can I use it instead of sand and aggregate?
Ballast is a pre-combined mix of sand and aggregate, common in the UK. It simplifies ordering since you buy one material instead of two, typically at a ratio of 1 part cement to 4 or 5 parts ballast depending on the strength needed, though you have less control over the exact sand-to-aggregate split.
What water-cement ratio should I use?
A ratio of about 0.5 works well for most general-purpose domestic mixes. Lower ratios, around 0.4 to 0.45, produce stronger concrete but are harder to place without a plasticizer additive.
What mix ratio should I use for a shed base?
A standard 1:2:4 mix is sufficient for most shed bases and other light, non-structural domestic work.
Does this calculator work in cubic metres and kilograms?
Yes. Switch to UK units and every field, including material weights and bag sizes, converts to metric automatically.