Concrete Calculator

1. Volume first, everything else after

Every concrete order starts as a volume, and every volume is length × width × depth with the units made to agree. The trap is that the depth is almost always given in inches or centimetres while the plan dimensions are in feet or metres, and a 4-inch slab entered as 4 feet is out by a factor of twelve.

Each dimension here carries its own unit dropdown, so a slab is entered as 20 ft × 10 ft × 4 in exactly as it is written on the drawing. Internally everything converts to cubic metres and back out once at the end, using the exact 1 yd³ = 0.764554857984 m³ rather than the 0.7646 in most reference tables.

For anything outside a pour — rebar weight in pounds against kilograms, a plan drawn in millimetres, an area quoted in square yards — the unit converter carries the same exact factors and the same habit of converting once rather than at every step.

The first three are the same rectangular prism under different labels — the calculator renames the fields rather than pretending they are different arithmetic. Use How many of these? for identical repeats: four footings or six columns are entered once and multiplied, which is both faster and harder to get wrong than adding four volumes by hand.

2. Stairs are not a wedge

A solid flight of stairs is the one shape people consistently underestimate. Its cross-section is a staircase, not a triangle, so the volume is rise × run × n(n + 1) ÷ 2 times the width — the triangular-number series, because step one is one rise deep, step two is two, and so on.

That series grows faster than the step count. Five steps at 7 in rise, 11 in run and 3 ft wide is 0.89 yd³; ten identical steps is 3.27 yd³ — not double but 3.7 times as much, because 55 units of rise × run instead of 15. Estimating stairs by scaling up a shorter flight is how a short load turns into a second delivery.

3. Waste, and why the ordered volume is never the drawing

The drawing gives an exact volume and the site never does. Sub-grade is not perfectly level, so a nominally 4-inch slab averages a little deeper. Forms bow outwards under the weight of wet concrete. Some is left in the barrow, the chute and the pump line. 10% is the usual allowance and it is the default here; tight forms on a prepared base can take 5%, and a rough excavation or a hand-dug footing can want 15% or more.

The Before waste and Waste added tiles keep both numbers on screen, because they answer different questions — one is what the structure contains, the other is what to order. On the 20 × 10 ft slab at 4 in that is 2.469 yd³ in the forms and 2.716 yd³ to order.

Then round up again at the supplier. Ready-mix is batched and sold in whole or half yards, and a short load fee for the extra half yard is cheaper than a cold joint where the truck ran out mid-pour. Running out is the one estimating error you cannot fix afterwards.

4. Bags or ready-mix, and where the crossover is

Bag counts use the yield printed on the bag, never the bag weight. An 80 lb bag of pre-mixed concrete makes about 0.60 ft³, because water and compacted paste account for part of the finished volume:

Counts are always rounded up, since two thirds of a bag is not a thing you can buy. That 20 × 10 ft slab needs 123 × 80 lb bags — 9,840 lb of material, close to five tons to carry, and a mixer running for most of a day. Past roughly 40 bags ready-mix is usually cheaper and always faster, and above about 1 yd³ it is the only realistic way to place a slab in one continuous pour.

With a price entered the Estimated cost tile multiplies your rate by the volume including waste — per cubic yard on imperial, per cubic metre on metric. It is the concrete only: delivery, short-load and out-of-hours surcharges, pump hire, formwork, reinforcement, labour and finishing are all extra, and on a small domestic pour they can exceed the concrete itself.

Labour is the line that most often dwarfs the material. If you are pricing your own time or a crew's, the hours calculator totals a week of clock-in and clock-out times with unpaid breaks taken off and overtime split out, which is the figure that belongs next to the cost above.

5. Mixing on site: nominal mixes and dry volume

For a hand or drum mix, the Mixing it yourself panel breaks the same volume into cement, sand, coarse aggregate and water at a nominal ratio:

Quantities are worked out on a dry volume of 1.54 × the wet volume. Sand and aggregate are full of voids that close up once cement paste and water fill them, so the dry ingredients always measure more than the finished pour — ignoring that factor is the classic reason a site mix comes up a third short. Cement is converted at its bulk density of 1,440 kg/m³ and reported both in 50 kg bags and in US 94 lb sacks, and water at a 0.50 water-cement ratio.

For 2.077 m³ of M20 that is roughly 837 kg of cement (17 × 50 kg or 20 × 94 lb), 0.87 m³ of sand, 1.74 m³ of aggregate and 419 litres of water. Add water sparingly: extra water makes concrete easier to place and measurably weaker, and a mix at w/c 0.65 can lose a third of the strength of the same mix at 0.45.