Material calculator
Concrete & cement calculator
Estimate concrete volume, cement bags, sand and crush for a rectangular slab or pour using transparent nominal-mix assumptions.
Calculation results
Formula and assumptions
The calculator finds rectangular wet volume, adds waste, multiplies by the editable dry-volume factor, then divides dry volume according to the selected cement:sand:aggregate ratio.
Wet cft = length × width × thickness ÷ 12
Dry cft = wet cft × (1 + waste %) × dry factor
Cement bags = cement share of dry cft ÷ 1.25
Nominal proportions are estimating aids. They do not replace a designed concrete mix, batching controls, cube tests or structural specifications.
Worked example
A 20 ft × 15 ft slab at 6 inches thick has 150 cft of wet concrete before waste. With 5% waste, a 1.54 dry factor and a 1:2:4 mix, the tool allocates the dry volume across seven total mix parts and converts the cement share into bags.
Measure the pour before estimating materials
Break an irregular pour into simple shapes and calculate each part separately. For a beam-and-slab system, measure slab panels, beams, columns and upstands from the approved drawings instead of multiplying the overall roof footprint by one thickness. Deduct only voids that are genuinely absent from the pour.
When the crushed-stone allowance needs a separate volume check, a rock and gravel calculator can help reconcile the aggregate quantity with this take-off.
- Use the same drawing revision for length, width and thickness.
- Keep cubic feet and cubic metres visible in the take-off so unit conversion can be checked.
- Record whether waste covers spillage only or also overbreak, uneven formwork and rejected material.
- Do not convert a nominal 1:2:4 result into a structural specification. Strength, workability, durability, aggregate moisture and trial batches belong to mix design and quality control.
Why the dry-volume factor is only an estimating convention
Loose ingredients do not translate to finished concrete through one universal multiplier. Aggregate grading, voids, moisture, compaction and batching method affect yield. ACI proportioning guidance uses absolute volumes and trial-batch adjustment, which is more rigorous than a fixed dry factor. This calculator keeps the factor editable because its purpose is early material planning, not mix approval.
Frequently asked questions
What does the dry-volume factor mean?
Dry ingredients occupy more loose volume than finished compacted concrete. The calculator defaults to 1.54 and lets you edit it for your estimating method.
How many cubic feet are assumed in one cement bag?
The calculator uses 1.25 cubic feet per bag as a planning conversion and shows this assumption beside the result.
Can this choose a structural concrete mix for me?
No. It only estimates materials for the mix you select. A qualified engineer must specify concrete strength and mix requirements.
Sources and verification
- American Concrete Institute — ACI PRC-211.1-22Concrete proportioning guidance based on absolute volume, workability, strength, durability and trial-batch adjustment.
- PSQCA PS 232-2015 — Ordinary Portland CementPakistan specification for Ordinary Portland Cement; use the applicable project specification and current standard edition.
- Pakistan Engineering Council — Building Code of Pakistan 2021National building-code context for safety, administration, structural work and the authority having jurisdiction.
Sources establish the technical context. Calculator planning assumptions remain separately labelled and should be checked for the specific project.
Need a project-wide planning estimate? Use the grey structure cost calculator. For reinforcement, use the steel / saria calculator.