How to use the Concrete Calculator
- 1 Enter the length and width in feet.
- 2 Enter the thickness in inches.
- 3 Choose the mix: 1:2:4, 1:1.5:3 or 1:3:6.
- 4 Read the cement, sand and crush.
Worked example
A 20 × 15 ft roof slab, 5 inches thick, in 1:2:4 concrete. The wet volume is 125 cft (about 3.5 m³). It needs about 22 bags of cement, 55 cft of sand and 110 cft of crush. Ask your engineer to confirm the slab thickness and mix before you order, because the design decides the real quantities.
Features
Common mixes
1:2:4 for slabs, 1:1.5:3 for columns, 1:3:6 for floors.
Dry volume
Wet volume × 1.54.
Metric too
Volume in cubic metres.
When to use it
Roof slabs
Order materials for a lintel or roof.
Floors
Plan a base concrete floor.
Columns and beams
Use a stronger mix.
Driveways
Estimate concrete for a garage floor.
Frequently asked questions
What is 1:2:4 concrete?
One part cement, two parts sand and four parts crush by volume, roughly M15 grade, used for slabs and beams.
Why multiply by 1.54?
Dry materials lose about a third of their volume when mixed with water, so 54% more dry material is needed.
How thick is a roof slab?
Usually 5 to 6 inches for houses in Pakistan, as designed by an engineer.
How much cement does one cft of concrete need?
About 0.18 bags for 1:2:4 concrete.
Should I use ready-mix?
Ready-mix concrete gives consistent quality for larger pours, especially in cities.
How long should concrete be cured?
At least 7 days for normal cement and 14 days for slabs in hot weather.
How much water is used in concrete?
Usually about half the weight of cement, adjusted for workability, as too much water weakens concrete.
Can I mix concrete by hand?
Small jobs can be hand mixed, but a mixer gives more uniform concrete.
When can I remove slab shuttering?
Usually after 14 to 21 days for slabs, as advised by your engineer.
What crush size should I use?
Three quarter inch crush is common for slabs and beams.
Does weather affect concrete?
Hot weather speeds up setting and cracking, so pour in the morning and cure well.
What does M15 or M20 mean?
These are concrete grades. M15 reaches about fifteen newtons per square millimetre after twenty-eight days and M20 about twenty, so M20 is stronger and used for columns and important members.
How many bags of cement are in a cubic metre of concrete?
For a 1:2:4 mix, about six to seven bags of cement are needed for each cubic metre of concrete.
Why must concrete be vibrated?
Vibrating removes trapped air so the concrete fills the formwork and surrounds the steel properly, giving a stronger and denser slab.
Results are estimates based on the official rates linked above. Confirm important figures with the issuing authority or a qualified adviser before you act on them.
Reviewed by ToolsUnlocked Team · Last updated