CalcLibrary

Concrete Calculator

Estimate how much concrete you need for a slab, footing, square column, or round column. Choose m³ or yd³ for the volume result, plus the exact bag count for 40 lb, 60 lb, 80 lb, or 25 kg pre-mix bags — with a built-in waste buffer so you don't run out mid-pour.

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Examples

A 4 m × 3 m × 0.10 m patio slab with 10% waste, 80 lb bags.

Volume Needed (with waste)
1.32 m³
Raw Volume (no waste)
1.2 m³
Bags Needed
78 80 lb bags

Plan on 78 bags. Pick them up before mixing — running short mid-pour is the #1 small-job mistake.

Estimate based on standard pre-mix yields (40 lb ≈ 0.30 cu ft, 60 lb ≈ 0.45 cu ft, 80 lb ≈ 0.60 cu ft, 25 kg ≈ 0.0125 m³). Always allow extra for spillage and over-excavation, and verify on the bag you actually buy.

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Examples

How It Works

Formula

Vslab=L⋅W⋅TV_{\text{slab}} = L \cdot W \cdot T

Vfooting=L⋅W⋅DV_{\text{footing}} = L \cdot W \cdot D

Vcolumn=W⋅D⋅HV_{\text{column}} = W \cdot D \cdot H

Vround=πr2HV_{\text{round}} = \pi r^2 H

Vpour=V⋅(1+w100)V_{\text{pour}} = V \cdot \left(1 + \tfrac{w}{100}\right)

N=⌈Vpour/y⌉N = \lceil V_{\text{pour}} / y \rceil

Variables

LL

Length

WW

Width (or column side)

TT

Slab thickness

DD

Depth (footings) or column side

HH

Column height

rr

Round-column radius

VV

Raw volume(m³)

ww

Waste percent

yy

Yield per bag(m³)

NN

Bags needed (rounded up)

Pick the shape and enter your dimensions in your preferred units. The calculator converts everything to metres, applies the standard volume formula for that shape, multiplies by 1 + waste/100 to get the pour quantity, converts to your selected volume unit, and divides by the per-bag yield (rounded up to whole bags).

Frequently Asked Questions

01How much waste should I add?
10% is the standard buffer for spillage, over-excavation, and small over-pours. Bump it to 15% for awkward shapes, hand-mixing on a windy site, or first-time DIY pours. Drop to 5% only when you can return unused ready-mix or are mixing bags one at a time.
02Which bag size is best?
80 lb (or 25 kg) bags give the best yield-per-bag and the lowest cost per cubic metre, but they are heavy. 60 lb bags are easier to carry and mix in a wheelbarrow. 40 lb bags are for very small patches. Above ~1 m³ (1.3 yd³), bagged concrete starts losing on price and labour to a ready-mix truck.
03How do I measure for a slab?
Length × width is the footprint. Thickness is the depth of the pour, not the gravel base. A typical patio is 4 in (10 cm); a driveway 4–6 in; a garage floor 4–6 in over a compacted gravel sub-base. Measure twice — concrete is unforgiving once it sets.
04What does 1 yard of concrete weigh?
1 cubic yard of standard concrete weighs roughly 4,050 lb (about 1,840 kg) at a typical 150 lb/ft³ density. 1 cubic metre weighs about 2,400 kg. Plan transport, formwork, and sub-base compaction for that load.
05Should I order extra concrete?
Yes. A small buffer helps cover spillage, uneven subgrade, and small form changes on site. Many pours add 5-10% on top of the measured volume, especially for footings, posts, and jobs where the base is not perfectly level.

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