WiseCalcs

Concrete calculator

Enter the length, width and thickness in metric or US customary units, set your own over-order allowance, and give the yield printed on your bag. You get the raw volume, the order volume, both of them in cubic yards as well, and the whole bags to buy.

Written by Kenneth Schrøder, Founder of WiseCalcs

Updated ·Sources verified

Volume is length × width × thickness. The order volume adds your allowance on top, and the bag count divides that by the mixed-concrete yield of one bag and rounds up. Every cubic-yard figure on the page says whether the allowance is in it.

m
m
cm
%

Your own margin for spillage, uneven sub-base and over-dig. 10% is a common starting point, not a rule.

Picking a size fills in the yield below. Choosing your own yield switches this to Custom.

L

Volume of wet concrete one bag makes, from the data sheet — not the dry weight. Sheets we checked give 0.43 and 0.50 m³ per tonne of dry mix, so a 25 kg bag lands somewhere near 10.8–12.5 L. Use your product's figure.

Order volume (allowance included)

1.32 m³

SlabAllowanceWhole-bag surplus
Before and after the allowance
m³yd³
Before allowance1.21.57
Order (with allowance)1.321.726
Whole bags
116
Concrete those bags make
1.322 m³

Quantities only. This does not size footings, reinforcement or anything load-bearing.

The result updates as you type. The headline is what to order, allowance included; the table underneath puts the raw pour and the order volume side by side in both units, so the two cubic-yard figures can never be confused. The bar shows how much of the concrete you buy is the pour, the allowance, and the surplus from bags being whole.

How does it work?

The cubic-yard figures are labelled before or after the allowance on the page, because the two differ by exactly the allowance you set. Bag counts come from the stated yield of mixed concrete, never from the dry weight on the bag: a 25 kg sack does not make 25 kg worth of volume, because the water fills the voids between the grains. Yields differ by product, so the field is editable.

Concrete volume and bag count

V=l×w×t,Vorder=V(1+s100),n=⌈Vordery⌉V = l \times w \times t,\quad V_{order} = V\left(1 + \tfrac{s}{100}\right),\quad n = \left\lceil \frac{V_{order}}{y} \right\rceil
V
Volume of the pour itself, before any allowance.
l, w
Length and width — metres, or feet in US mode.
t
Thickness — centimetres, or inches in US mode. Converted to the same unit as l and w before multiplying.
s
Your waste / over-order allowance, in percent.
V order
What to order: the pour plus the allowance.
y
Mixed concrete one bag makes, from the product data sheet.
n
Whole bags. Always rounded up, because part of a bag still costs a bag.

Metric: a 4 m × 3 m slab 10 cm thick is 1.2 m³. A 10% allowance makes the order volume 1.32 m³ (1.726 yd³), and at 11.4 L a bag that is 116 whole bags. US: a 12 ft × 10 ft slab 4 in thick is 40 ft³ (1.481 yd³); with 10% the order volume is 44 ft³ (1.630 yd³), which is 74 whole 80 lb bags at the 0.60 ft³ yield QUIKRETE states.

Method & sources

Metric mode takes metres and centimetres; US customary mode takes feet and inches. Switching converts the numbers, so the slab stays the same size. The waste / over-order allowance is yours to set. It starts at 10%, which is a common starting point rather than a rule. Bag counts come from the yield of mixed concrete you enter, not from the bag's dry weight, and round up to whole bags.

Sources

Where this method comes from — use these references to understand the formula, assumptions, and limits.

How we calculate

  • Metric mode takes metres and centimetres; US customary mode takes feet and inches. Switching converts the numbers, so the slab stays the same size.
  • The waste / over-order allowance is yours to set. It starts at 10%, which is a common starting point rather than a rule.
  • Bag counts come from the yield of mixed concrete you enter, not from the bag's dry weight, and round up to whole bags.
  • The default 11.4 L for a 25 kg bag is a placeholder between two published figures (0.43 and 0.50 m³ per tonne of dry mix). Replace it with your own product's.
  • The 40, 60 and 80 lb presets are the yields QUIKRETE states for its Concrete Mix: 0.30, 0.45 and 0.60 ft³.
  • The slab is treated as a simple rectangular block of even thickness.
  • Quantities only. Nothing here sizes footings, reinforcement or any load-bearing element.

Rounding

Volumes are shown to three decimals and bag counts as whole bags. The calculation uses full precision.

What this calculator does

Concrete is ordered and mixed by volume. This calculator multiplies length, width and thickness into a volume, adds an allowance you choose, converts both figures into the other volume unit, and turns the order volume into whole bags. It works in metres and centimetres or in feet and inches, and switching between them converts the numbers rather than reinterpreting them, so the slab does not change size when you change units.

How to use it

  1. Pick metric or US customary units.
  2. Enter the length and width, then the thickness — centimetres in metric, inches in US.
  3. Set the over-order allowance. 10% is the starting value, not a rule; a rough sub-base deserves more.
  4. Choose your bag size, or type the yield of mixed concrete printed on the bag.
  5. Order the volume that includes the allowance, and buy the whole-bag figure.

A worked example

Metric: a 4 m × 3 m slab 10 cm thick is 12 m² × 0.1 m = 1.2 m³, which is 1.57 yd³. A 10% allowance makes the order volume 1.32 m³, or 1.726 yd³ — the same slab, after the allowance rather than before it. At 11.4 L a bag that is 116 whole bags. US: a 12 ft × 10 ft slab 4 in thick is 40 ft³, or 1.481 yd³. With the same 10% allowance the order volume is 44 ft³, or 1.630 yd³, and at the 0.60 ft³ yield QUIKRETE states for an 80 lb bag that is 74 whole bags.

Bags or ready-mix?

Bags suit small jobs; above roughly half a cubic metre, ready-mix delivered by volume is usually cheaper, faster, and avoids mixing a large pour in batches. The bag count here is only as good as the yield you give it — and the yield is the figure to look for, not the weight. A 25 kg bag does not make 25 kg worth of volume: the water fills the voids between the grains, so the mixed material takes up less room than the dry ingredients. Two European data sheets we checked state 0.43 and 0.50 cubic metres per tonne of dry mix, which is about 10.8 and 12.5 litres from a 25 kg bag — a difference of one bag in seven on the same job.

Common mistakes

  • Entering thickness in the wrong unit — metres instead of centimetres, or feet instead of inches.
  • Reading a cubic-yard figure without checking whether the allowance is in it. The page labels both.
  • Scaling up the bag's dry weight instead of its stated yield of mixed concrete.
  • Ordering exactly the raw volume and leaving no allowance at all.
  • Treating a sloped or thickened-edge slab as a flat block.

When it's useful

Planning a patio, shed base, path, or footing — anywhere you need to know the concrete volume and roughly how much to buy before starting.

FAQ

How is the volume calculated?
Length times width times thickness, with the thickness brought to the same unit as the other two first. A 4 × 3 m slab 10 cm thick is 1.2 m³; a 12 × 10 ft slab 4 in thick is 40 ft³. That is the raw pour, before any allowance.
Why add a waste allowance?
Some concrete is lost to spillage, over-dig and over-filled forms. The field starts at 10%, which is a common starting point rather than a rule — raise it on rough sub-grade, lower it on a tidy, formed pour. It is your assumption, and the page shows the volume with and without it.
How many bags will I need?
From the yield you enter, not the bag's weight. Pick a bag size to fill the field in, or type your own: QUIKRETE states 0.30, 0.45 and 0.60 ft³ for its 40, 60 and 80 lb bags, and the 11.4 L default for a 25 kg bag sits between the 0.43 and 0.50 m³ per tonne two European sheets state. Yields differ enough between products to change the bag count, so the number on your own bag is the one to use.
Does it handle non-rectangular slabs?
It calculates a rectangle. For L-shapes, split the area into rectangles, calculate each, and add the volumes.
What thickness should a slab be?
It depends on the load: paths are often 10 cm, while driveways and heavier loads need more. Follow local building guidance.
Can I share a calculation?
Yes. Use Share to copy a link that reopens the calculator with the same dimensions.

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