How much waste should I allow?

10% for a slab on well-prepared ground, 15% if the subgrade is uneven or you are pouring into trenches where over-dig is inevitable. Running out mid-pour is far worse than a little left over, since a cold joint through the middle of a slab is a permanent weakness.

At what point should I order ready-mix instead of bags?

Around half a cubic metre — about 40 bags. Below that, bags are convenient and the joints do not matter much. Above it, the labour becomes serious and the quality genuinely suffers from mixing in batches.

Concrete Calculator

Calculate how much concrete you need for foundations, slab, or other construction.

Length (m)

Width (m)

Depth (m)

Estimating concrete, and when to stop using bags

Concrete volume is length times width times depth — the easy part. What the calculation really tells you is whether this is a bag job or a ready-mix job, and that threshold arrives sooner than most people expect.

How it works

Multiplies the three dimensions to give volume in cubic metres.
Adds a waste allowance for spillage, uneven subgrade and over-dig.
Converts the result into bags and into ready-mix volume, so the two options can be compared directly.

volume = length × width × depth   (all in metres)
ordered = volume × (1 + waste)

25 kg bag yields roughly 0.0125 m³
bags = ordered ÷ 0.0125

ready-mix is sold in 0.5 m³ steps, so round up

Worked example

A 5 × 3 m shed base, 100 mm deep, with a 10% waste allowance.

volume = 5 × 3 × 0.1 = 1.5 m³
with 10% waste = 1.65 m³
as bags: 1.65 ÷ 0.0125 = 132 bags
132 × 25 kg = 3.3 tonnes to carry and mix by hand
as ready-mix: round up to 2.0 m³

132 bags is over three tonnes — several days of mixing and a base with visible cold joints. At 1.5 m³ this is firmly ready-mix territory; the practical crossover is around 0.5 m³.

Reading the result

Above roughly 0.5 m³, ready-mix wins on cost, time and quality. Hand-mixed batches cure at different rates and never bond as one slab, which is why large hand-poured bases crack along the joins.
Depth is where the error usually is. A base dug 20 mm deeper than planned over 15 m² is an extra 0.3 m³ — 20% more concrete than you ordered. Check the depth in several places before ordering, not just at one corner.
Ready-mix arrives on a schedule and does not wait. Formwork, reinforcement and a route for the barrow all have to be finished before the truck arrives, and you need more hands than you think.
The 0.0125 m³ per 25 kg bag is nominal. Yield varies by mix and by how much water you add, so treat the bag count as a minimum rather than an exact figure.

Common questions

How much waste should I allow?: 10% for a slab on well-prepared ground, 15% if the subgrade is uneven or you are pouring into trenches where over-dig is inevitable. Running out mid-pour is far worse than a little left over, since a cold joint through the middle of a slab is a permanent weakness.
At what point should I order ready-mix instead of bags?: Around half a cubic metre — about 40 bags. Below that, bags are convenient and the joints do not matter much. Above it, the labour becomes serious and the quality genuinely suffers from mixing in batches.