Concrete Calculator

What is this concrete calculator for?

This tool estimates the concrete volume required for a slab, driveway or footing, along with an approximate material split based on a chosen mix ratio.

It also converts cement mass to bags and gives an estimated cost if you provide a price per bag.

Formulas used

Volume

Volume = length × width × thickness. For common shapes: slab/footing V = L × W × T; cylinder V = π × (D/2)^2 × H; annulus (tube) V = π × (R_o^2 − R_i^2) × H. Use annulus when the element is hollow (tube/sonotube), cylinder when solid. We apply a dry‑volume factor (~1.54) to offset voids/segregation. The tool converts units and rounds to 2 decimals for readability; for ordering, round up. Tip: add 5–10% contingency for waste and joints.

Mix ratios

1:2:4 or 1:3:6 represent cement:sand:gravel. Density assumptions (approx.): cement 1440 kg/m³, sand 1600 kg/m³, gravel 1500 kg/m³, water 1000 kg/m³. Water is estimated with W/C ≈ 0.5 (adjust for workability). These are averages; adjust for aggregate grading, exposure class and target slump.

Bags conversion

Bags = cement mass ÷ bag size. Example: 420 kg of cement with 35 kg bags ⇒ 12 bags (round up). Typical sizes: 25/35/50 kg (metric), 47/60/94 lb (US). Always round up and consider 1–2 extra bags as contingency on small jobs.

Worked examples

Enable comparison B to contrast scenarios and visualize material distribution.

Benefits and limitations

Benefits

• Faster estimates with fewer manual mistakes.
• Sharper purchasing and logistics planning (bags, pallets, lead‑times and delivery windows).
• Quick scenario comparisons (thickness, ratios, units).

Limitations

• Approximations due to densities, mix ratios and rounding.
• Does not replace a professional quote or structural design; local codes may impose minimums.
• Site variability: sub‑base compaction, reinforcement, target slump and losses can change actual volume.

Practical tips

Add a margin (5–10%) for losses and check local recommendations for thickness and concrete grade.