Ordering concrete is a one shot decision. The truck arrives, the chute swings out, and whatever is in the drum is what you place. A short load means a cold joint across the middle of the pad, and an over order means paying for material that goes to the spoil pile. The arithmetic here stays deliberately plain so the argument moves to where it belongs: the depth you actually dug, the waste allowance you can defend, and the unit your supplier bills in. A 5 m by 2 m pad at 100 mm is 1 cubic meter on paper and 1.05 cubic meters once a 5 percent allowance is added, which is already past the point where bagged product stops being sensible.
Measuring the hole, not the drawing
- Set the finished level first. Drive a peg at the highest point of the excavation and mark the finished surface height on it, then transfer that mark to a peg at every corner using a laser level or a straight board and spirit level. Every depth you record later is measured down from this single line, never from the ground surface. Tool: Laser level or spirit level.
- Measure length and width at the forms. Run the tape along the inside face of the formwork at three positions on each axis, at both ends and at the middle, and record the largest figure in meters to two decimal places. Timber forms bow outward under the weight of wet concrete, so the widest reading is the honest one. Tool: Steel tape.
- Probe the depth on a grid. Lay a straight edge across the forms and measure down to the compacted sub base at every meter in both directions, writing each figure in millimeters. On a 5 m by 2 m pad that is eighteen readings, and the average of those readings is the depth you type in, not the design depth. Tool: Straight edge and folding rule.
- Add thickened edges and returns. Measure any perimeter thickening, footing beam, or step down as its own length, width, and depth, and take that depth below the underside of the slab so the slab volume is never counted twice. A 200 mm wide edge dropped 50 mm under a 100 mm slab follows a 13.2 m center line on this pad and adds 0.13 cubic meters. Tool: Steel tape.
- Choose a waste allowance you can justify. Write down the reason for the allowance before you type it, because 5 percent on this pad is 0.05 cubic meters and 15 percent is 0.15 cubic meters. Rigid forms on a level compacted sub base justify the low end, and a hand dug bottom in soft ground justifies the high end.
A 5 m by 2 m pad poured at 100 mm
| Field | Value |
|---|---|
| length | 5 |
| width | 2 |
| depth | 0.1 |
| waste | 5 |
The pad measured 5.00 m along the forms and 2.00 m across, and the eighteen depth probes averaged 0.10 m against a design depth of 100 mm. Multiplying those three figures gives exactly 1 cubic meter of neat volume. The 5 percent allowance adds 0.05 cubic meters, so the tool reports 1.05. Length, width, and depth each enter the volume once, so a 10 percent change to any one changes the unrounded estimate by 10 percent when the others stay fixed. Dropping the depth average to 0.09 m cuts the order by 10 percent to 0.945 cubic meters, while an over dug 0.11 m average raises it by 10 percent to 1.155. The largest real uncertainty depends on which site measurement varies most. Converted with the exact NIST factor of 0.7645549 cubic meters per cubic yard, 1.05 is 1.37 cubic yards, and at 0.02831685 cubic meters per cubic foot it is 37.08 cubic feet. Nothing is rounded until that final step.
Result: 1.05 cubic metres.
Why depth decides more than length or width
Length and width are set by the forms and are hard to get wrong by more than a few centimeters. Depth is set by the ground, and the ground does not cooperate. A 5 m by 2 m pad covers 10 square meters, so every millimeter of extra excavation across that area is another 0.01 cubic meters of concrete. Ten millimeters of over dig, less than the thickness of a pencil laid flat, adds 0.1 cubic meters, roughly a tenth of the whole order. The residential code sets 3 1/2 inches, or 89 mm, as the minimum for a slab on ground floor, and the 100 mm sitting in the depth field by default is 11 mm above that line. That margin exists because sub bases settle and screeds drift. It is not spare material to shave off when the volume looks awkward, and it is the reason the depth figure has to be an average of many probes rather than one confident reading taken at the corner where the tape was easiest to hold.
| Bag size | Yield per bag | Liters per bag | Bags per cubic meter | Water per bag, start to maximum |
|---|---|---|---|---|
| 40 lb (18.1 kg) | 0.30 cu ft | 8.5 L | 118 | 3 pt to 4.5 pt (1.4 L to 2.1 L) |
| 50 lb (22.6 kg) | 0.375 cu ft | 10.6 L | 95 | 3.5 pt to 5.5 pt (1.7 L to 2.6 L) |
| 60 lb (27.2 kg) | 0.45 cu ft | 12.7 L | 79 | 4 pt to 7 pt (1.9 L to 3.3 L) |
| 80 lb (36.2 kg) | 0.60 cu ft | 17 L | 59 | 6 pt to 9 pt (2.8 L to 4.3 L) |
| 90 lb (40.8 kg), regional | 0.675 cu ft | 19.1 L | 53 | 7 pt to 10 pt (3.3 L to 4.7 L) |
Source: The Quikrete Companies, Concrete Mix Product No. 1101 Technical Data Sheet.
Bag sizes, yields, liter equivalents, and mixing water are printed on the product data sheet. The bags per cubic meter column is arithmetic from those liter yields, rounded up to the next whole bag.
Reading the number as bags, barrows, and trucks
A result of 1.05 cubic meters is a purchasing decision in three different currencies. As bagged product it is 1,050 liters divided by the 17 liter yield of an 80 lb bag, which comes to 62 bags weighing about 2,244 kg, and the same data sheet asks for at least 1 cubic foot of mixer capacity per 80 lb bag, so the mixer on hand, not the arithmetic, decides how many bags go in at one time. As ready mix it is 1.37 cubic yards using the exact factor of 0.7645549 cubic meters per cubic yard, and most suppliers round that up on the ticket. As mass it is roughly 2,355 kg at the 2,242.5 kg per cubic meter unit weight printed on the same sheet, which matters when the only access to the bed is a wheelbarrow ramp. The volume never changes. The unit it is sold in does, and ordering mistakes happen at that conversion far more often than in the multiplication.
Ground conditions that quietly add volume
Concrete finds every soft spot. Where the bed was built up as fill rather than cut from undisturbed ground, the residential code limits that fill to 24 inches, or 610 mm, for clean sand and gravel and to 8 inches, or 203 mm, for earth, and anything looser keeps compacting under the weight of the pour. Wheel ruts left by a barrow crossing the bed are commonly 20 mm to 40 mm deep, and they fill with concrete rather than air. Timber forms bow outward the moment the drum tips, which is why the width belongs measured at the middle of the run as well as at the ends. Where the bed slopes, the low corner controls the volume, and averaging only the corner probes will understate it. None of this is unusual site behavior, and none of it is covered by a waste percentage chosen before the excavation was inspected. Walk the bed with a straight edge on the morning of the pour and correct the depth figure then.
Common mistakes
- Ordering from the design depth instead of the measured average depth.
- Excavation almost never lands on the drawing. A sub base that dips 15 mm below target across a 10 square meter pad hides an extra 0.15 cubic meters, which is 15 percent of a 100 mm slab and more than most waste allowances were sized to cover. Probe the depth on a grid, average every reading, and type that average into the field. If the average sits more than 10 mm off the design depth, correct the sub base before ordering rather than paying for the difference in concrete.
- Treating bagged mix and ready mix as interchangeable at any size.
- The data sheet asks that a form be filled to its full depth and worked continuously from one end to the other, because partial depth fills leave cold joints. Hand mixing anything past about a quarter of a cubic meter turns that single continuous placement into a queue of separate batches. Keep bagged product for footings, posts, and repairs under about 0.25 cubic meters, which is fifteen 80 lb bags. Above that, price a ready mix delivery and ask the supplier what their smallest billed load actually is.
- Adding a waste percentage on top of a depth that was already rounded up.
- Rounding a 0.095 m average up to 0.10 m has already added about 5 percent. Stacking a 5 percent allowance on that carries close to 10 percent of hidden overage, which on a 1 cubic meter pad is about 0.10 cubic meters paid for twice. Keep the measured average at the precision you genuinely read it, to the nearest 5 mm, and let the waste field carry the entire allowance. One rounding step, applied once, at the very end of the calculation.
- Pouring thinner than the code minimum so the ordered volume fits.
- The residential code sets 3 1/2 inches, or 89 mm, as the minimum for slab on ground floors and points to ACI 332 for everything beyond that. Shaving the last 10 mm to dodge a second delivery is the one saving that fails inspection. Order the volume that the measured depth demands and treat a part load charge as the price of a compliant slab. If the pad is a floor rather than a garden path, confirm the required thickness with the building department first.
When a supplier or the building department takes over
Volume math has a boundary and it is worth naming out loud. The packaged mix cited here reaches about 2,500 psi at 7 days and 4,000 psi at 28 days when tested to ASTM C39, but nothing in a volume estimate says whether that strength, that thickness, or that reinforcement suits what will stand on the slab. Anything carrying a structural load, including a wall, a column, a retaining face, or a footing under habitable space, belongs to the local code and to ACI 332 rather than to a tape measure. Weather is the other boundary. The data sheet asks that fresh concrete be protected from freezing for the first 48 hours and warns against curing compounds when rain or temperatures below 50 degrees Fahrenheit are expected within 24 hours, with curing running 5 days at 70 degrees Fahrenheit or warmer and 7 days between 50 and 70. Order the volume yourself, let the supplier specify the mix, and let the building department confirm the thickness.
Frequently asked questions
How much concrete does a 5 m by 2 m pad at 100 mm need?
One cubic meter before any allowance, and 1.05 cubic meters with a 5 percent waste factor. That is 1.37 cubic yards or 37.08 cubic feet if your supplier bills in imperial units. The neat figure is length times width times average depth, so 5 times 2 times 0.1 gives exactly 1. Everything above that number is the allowance you chose, not the geometry of the pad.
Is a 5 percent waste allowance enough for a slab?
It is reasonable for a rectangular pad with rigid forms sitting on a compacted, screeded sub base, where the only real losses are chute spillage and a slightly uneven bottom. Move to 10 percent when the excavation was dug by hand, when the sub base is soft, or when the shape has returns and thickened edges. The allowance is a measurement confidence figure, not a fixed rule.
How many 80 lb bags replace one cubic meter of ready mix?
Fifty nine, because an 80 lb bag yields about 0.60 cubic feet, which the manufacturer lists as 17 liters, and a cubic meter is 1,000 liters. At 36.2 kg per bag that is over 2,100 kg to carry and mix by hand. The same sheet asks for 1 cubic foot of mixer capacity per 80 lb bag, so a small barrel mixer would run dozens of consecutive batches.
Why is depth harder to measure than length and width?
Length and width are read against a rigid form face, so the tape has something solid to press against and two people will write down the same figure. Depth is read against a compacted sub base that gives under a boot, keeps barrow ruts, and dips wherever the bed was built up as fill rather than cut from undisturbed ground. That makes it a measurement problem rather than an arithmetic one. The answer is more readings, taken to the nearest 5 mm and averaged, before any number is typed into the field.
Does this result account for reinforcement and footings?
No. Steel displaces a small amount of concrete, but the larger omission is geometry. Perimeter thickenings, footing beams, steps, and haunches are separate rectangular prisms that you measure and add yourself. Run the tool once for the slab and once for each additional element, then total the results before you call the supplier.
Related calculators
Sources
- Concrete Mix Product No. 1101 Technical Data Sheet The Quikrete Companies. Supplies the bag sizes, yields, liter equivalents, and mixing water figures in the table, plus the 2,242.5 kg per cubic meter unit weight, the 1 cubic foot of mixer capacity per 80 lb bag, the 2,500 psi and 4,000 psi strengths, and the freezing and curing periods quoted in the text. Accessed 2026-09-06.
- 2021 International Residential Code, Chapter 5 Floors International Code Council. Source of the 3 1/2 inch, or 89 mm, minimum slab on ground thickness, the reference to ACI 332, and the 24 inch and 8 inch fill depth limits cited in the ground conditions section. Accessed 2026-09-06.
- NIST Guide to the SI, Appendix B.9 Factors for Units National Institute of Standards and Technology. Provides the exact conversion factors used on this page, 0.7645549 cubic meters per cubic yard and 0.02831685 cubic meters per cubic foot, which turn the metric result into the units most ready mix suppliers quote. Accessed 2026-09-06.
This is a planning result, not a supplier quote; sellers use different pack, bag, board, roll, and bundle conventions, so confirm the exact product label and selling unit before you order.