Concrete Bag Calculator
Stopping at the volume is where these jobs go wrong. Yield per bag differs by product, so it is an input here instead of a constant — a 40 kg premix bag gives about 18 L, a 20 kg bag about 9 L. A 4 × 3 m pad at 10 cm is 1.2 m³, or 1,200 L; add 10% waste for 1,320 L and divide by 18 L to get 73.3, so you buy 74 bags. Bags are not sold in fractions, so the count always rounds up.
Your numbers
Bags needed
74bags
Volume
1.2m³
Litres
1,320L
Formula
Bags needed = ⌈Length × Width × Thickness ÷ 100 × 1000 × (1 + Waste allowance ÷ 100) ÷ Yield per bag (L)⌉
Yields vary between brands — copy the figure printed on the bag. Concrete cannot be revived once it starts to set, so on a slab poured in one go, running short costs far more than a spare bag does.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Length (m)The measurement across, side to side. | 4 | 0 and up |
| Width (m)The measurement front to back. | 3 | 0 and up |
| Thickness (cm)How thick one layer is; times the number of layers gives the whole. | 10 | 0 and up |
| Yield per bag (L)The volume one bag makes once mixed — copy the yield printed on the bag. | 18 | 0 and up |
| Waste allowance (%)The extra bought to cover breakage and offcuts. | 10 | 0 ~ 50 |
Step by step
Quick reference table
Results when only Length (m) changes and everything else stays put.
| Length (m) | Bags needed (bags) | Volume (m³) | Litres (L) |
|---|---|---|---|
| 2 | 37 | 0.6 | 660 |
| 3 | 56 | 0.9 | 990 |
| 4 | 74 | 1.2 | 1,320 |
| 6 | 111 | 1.8 | 1,980 |
| 8 | 147 | 2.4 | 2,640 |
What each result means
| Result | At default values |
|---|---|
| Bags needed (bags)How many bags to buy; they are not sold in fractions, so this rounds up. | 74 |
| Volume (m³)The space a solid holds — length cubed. | 1.2 |
| Litres (L)Capacity in litres; one litre is 1,000 cm³. | 1,320 |
Common mistakes
Yields vary between brands — copy the figure printed on the bag. Concrete cannot be revived once it starts to set, so on a slab poured in one go, running short costs far more than a spare bag does.
Glossary
- Length
- The measurement across, side to side.
- Width
- The measurement front to back.
- Thickness
- How thick one layer is; times the number of layers gives the whole.
- Yield per bag (L)
- The volume one bag makes once mixed — copy the yield printed on the bag.
- Waste allowance
- The extra bought to cover breakage and offcuts.
- Bags needed
- How many bags to buy; they are not sold in fractions, so this rounds up.
- Volume
- The space a solid holds — length cubed.
- Litres
- Capacity in litres; one litre is 1,000 cm³.
Frequently asked questions
Q. How is Concrete Bag Calculator calculated?
Bags needed = ⌈Length × Width × Thickness ÷ 100 × 1000 × (1 + Waste allowance ÷ 100) ÷ Yield per bag (L)⌉ — Stopping at the volume is where these jobs go wrong. Yield per bag differs by product, so it is an input here instead of a constant — a 40 kg premix bag gives about 18 L, a 20 kg bag about 9 L. A 4 × 3 m pad at 10 cm is 1.2 m³, or 1,200 L; add 10% waste for 1,320 L and divide by 18 L to get 73.3, so you buy 74 bags. Bags are not sold in fractions, so the count always rounds up.
Q. Can you walk through an example?
With Length 4m, Width 3m, Thickness 10cm, Yield per bag (L) 18, Waste allowance 10%, the answer is Bags needed 74bags.
Q. What do I need to enter?
Enter Length, Width, Thickness, Yield per bag (L), Waste allowance. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Length (m) moves the answer to Length (m) 2 → Bags needed (bags) 37 and Length (m) 8 → Bags needed (bags) 147. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
Yields vary between brands — copy the figure printed on the bag. Concrete cannot be revived once it starts to set, so on a slab poured in one go, running short costs far more than a spare bag does.
Related calculators
Keep your length units consistent across inputs — mixing centimetres and metres throws results far off.