Concrete Mathematics and Formwork Calculations
Reviewed by a working tradesman with 17+ years in the field.
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How much concrete to order and how much form material to build with, worked out cleanly and checked twice.
Tap your level and the boxes below open to the advice for you. Remembered on this device.
By the end you can
- Work out the volume of a slab, footing, wall, column and set of stairs.
- Add the right waste allowance and round the order sensibly.
- Count form boards, ties and whaler rows for a take-off.
- Say what the concrete plant needs to know when you order.
- Spot the common mistakes that leave you short.
Formwork is a worker-safety matter under your provincial OHS regulation. A form blowout can kill someone. Tie spacing and whaler spacing come from the form maker's tables or the engineer's design, not from a general rule. If you are building tall walls or unusual forms, ask the form supplier or engineer what applies in your province. The take-off formulas on this page are for ordering only.
Volume basics
All concrete volume is length × width × height, in the same units. Work in metres. Convert millimetres to metres first: 100 mm is 0.1 m.
| Conversion | Value |
|---|---|
| 1 m³ | 35.3 ft³ |
| 1 yd³ | 27 ft³ = 0.765 m³ |
| 1 m³ | 1000 litres |
| Normal concrete weighs about | 2400 kg per m³ |
Concrete weight varies with the mix. Use about 2400 kg/m³ for a quick check, and the plant's number for anything critical.
Slabs, footings and walls
A 6 m × 6 m slab, 100 mm thick. 100 mm = 0.1 m. Volume = 6 × 6 × 0.1 = 3.6 m³. Add 10% waste: 3.6 × 1.10 = 3.96 m³. Order about 4.0 m³.
Imperial check: a 20 ft × 20 ft slab at 4 in is 4.94 yd³, about 3.8 m³.
30 m of footing, 400 mm wide, 200 mm deep. Volume = 30 × 0.4 × 0.2 = 2.4 m³. Add 15% waste: 2.4 × 1.15 = 2.76 m³. Order about 2.8 m³. Footings poured against rough dug ground use more concrete than the drawing says, so add the waste.
12 m long, 2.4 m high, 200 mm thick. Volume = 12 × 2.4 × 0.2 = 5.76 m³. Add 15% waste: 5.76 × 1.15 = 6.62 m³. Order about 6.7 m³. Subtract big openings only if they are large. Small windows and sleeves are not worth the risk of coming up short.
The same math in feet and inches
Many crews think in feet, inches and yards. Ready-mix is sold by the cubic metre in Canada, but plenty of people still say yards. The method is identical. Convert every dimension to feet first (divide inches by 12), multiply to get cubic feet, then divide by 27 for cubic yards.
16 in = 1.333 ft and 8 in = 0.667 ft. Volume = 100 × 1.333 × 0.667 = 88.9 ft³. Divide by 27: 3.29 yd³. Add 15% waste: 3.79 yd³. Order about 3.8 yd³, which is about 2.9 m³.
8 in = 0.667 ft. Volume = 40 × 8 × 0.667 = 213.3 ft³. Divide by 27: 7.90 yd³. Add 15% waste: 9.09 yd³. Order about 9.1 yd³, which is about 7.0 m³.
4 in = 0.333 ft. Volume = 20 × 20 × 0.333 = 133.3 ft³. Divide by 27: 4.94 yd³. Add 10% waste: 5.43 yd³. Order about 5.4 yd³, which is about 4.2 m³.
Quick conversions: 1 yd³ = 0.765 m³ and 1 m³ = 1.308 yd³. A 2.44 m (8 ft) form board and a 4.88 m (16 ft) whaler plank are the same lumber in either language. For tubes, an 8 in tube is about 0.032 m³ per metre, or 0.35 ft³ per foot.
Tell the plant which unit you are ordering in. A mix-up between yards and cubic metres is a 31% error.
Columns and piers
Round column volume = π × radius² × height. A 300 mm diameter column, 3 m high: radius 0.15 m. Volume = 3.14 × 0.15² × 3 = 0.21 m³. Add 10%: 0.23 m³. Order about 0.25 m³.
Fibre tube quick reference
Volume per metre of tube height:
| Tube diameter | Volume per metre of height |
|---|---|
| 200 mm (8 in) | 0.031 m³ |
| 250 mm (10 in) | 0.049 m³ |
| 300 mm (12 in) | 0.071 m³ |
| 350 mm (14 in) | 0.096 m³ |
| 400 mm (16 in) | 0.126 m³ |
Stairs
The volume depends on how the stairs are poured. For solid concrete steps poured up from the ground, each step is a block as tall as everything under it.
Example: 4 solid steps, 250 mm tread, 900 mm wide, 180 mm riser. One riser-high block is 0.25 × 0.9 × 0.18 = 0.0405 m³. The four steps stack to 10 blocks, so 0.0405 × 10 = 0.41 m³. Add 10 to 15% waste and order about 0.45 to 0.5 m³.
Sidewalks, curbs and bagged concrete
- Sidewalk: length × width × thickness.
- Curb: length × height × thickness, plus the cross-section if it is shaped.
- Bagged concrete: bags needed = total volume ÷ the yield printed on the bag. Yields differ by product, so use the yield on the bag you are buying, in the same unit as your volume.
Formwork take-off
A formwork take-off is a count of boards, ties, whalers and bracing. Many suppliers will do it for you. Learn to check theirs.
30 m of footing forms, both sides, one course, 2.44 m (8 ft) boards. 30 ÷ 2.44 = 12.3, round up to 13 per side. Two sides: 13 × 2 = 26 boards. Add 15% for cuts and damage: 26 × 1.15 = 29.9. About 30 boards. Counting one side only would leave you about half short.
Ties ≈ (wall length ÷ horizontal spacing) × (wall height ÷ vertical spacing). A 12 m × 2.4 m wall at 600 mm × 600 mm: 12 ÷ 0.6 = 20 and 2.4 ÷ 0.6 = 4, so 20 × 4 = 80 as a base figure. Add ties for the ends, corners and openings, plus 5 to 10% spare, and you are at about 90. The real spacing comes from the form maker's tables or the engineer's design.
Whaler rows = wall height ÷ vertical spacing. Total whaler length = rows × wall length × sides. A 12 m × 2.4 m wall, 800 mm row spacing, both sides: 2.4 ÷ 0.8 = 3 rows per side, so 6 rows. 6 × 12 m = 72 m of whaler. With 4.88 m (16 ft) lumber: 72 ÷ 4.88 = 14.8, so about 15 pieces, plus 10% for laps and cuts.
Estimating allowances
| Item | Typical allowance |
|---|---|
| Slabs | 5 to 10%, up to 15% on uneven subgrade |
| Footings | 10 to 15% |
| Walls | 10 to 20% |
| Form boards | 10 to 20% |
| Ties | 5 to 10% |
| Whalers | about 10% |
These are typical estimating allowances, not code values. Adjust for your site, and ask your supplier and your lead what they use.
What to tell the concrete plant
- Volume in m³, including your waste allowance
- Strength (MPa) and exposure class as shown on the drawings. Concrete in Canada is specified under CSA A23.1.
- Air entrainment, slump and any admixtures the project calls for
- Placement method (chute, pump, bucket) and site access
Strength, exposure class and air content come from the drawings. The model code sets minimums for plain houses, and the drawings can ask for more:
| Part of the house | Minimum 28-day strength | Air | Max water ratio |
|---|---|---|---|
| Footings, foundation walls, columns, piers | 15 MPa | None set | 0.70 |
| Floors other than garage and carport | 20 MPa | None set | 0.65 |
| Garage and carport floors, exterior steps | 32 MPa (30 MPa where local aggregates cannot reach 32) | 5 to 8% | 0.45 |
Max water ratio is water to cementing material for site-batched concrete. From NBC 9.3.1.6 and 9.3.1.7 as printed in the BC Building Code 2012 (unreinforced concrete). Check your province's current edition. The drawings and the engineer can require higher values, and slump and delivery are covered in the Concrete Delivery and Slump lessons.
Worked example: the order for a small foundation
A foundation has 30 m of footing at 400 × 200 mm and a 12 m wall, 2.4 m high, 200 mm thick. What do you order?
- Footing. 30 × 0.4 × 0.2 = 2.4 m³. With 15% waste: 2.76 m³.
- Wall. 12 × 2.4 × 0.2 = 5.76 m³. With 15% waste: 6.62 m³.
- Total. 2.76 + 6.62 = 9.38 m³. Round up and order 9.4 m³.
- Check it a second way. Without waste, 2.4 + 5.76 = 8.16 m³. Waste added is 9.38 − 8.16 = 1.22 m³, about 15%. The two methods agree.
- Weight. At about 2400 kg/m³, 9.4 m³ is about 22.6 tonnes. That is how heavy the forms and the ground have to carry.
- Tell the plant. Volume, strength and exposure class from the drawings, slump, air, how it will be placed, and the access.
Common mistakes
- Mixing millimetres and metres. Convert first.
- Counting one side of the forms. The count is for every side.
- No waste on a footing against rough ground.
- Using ties or whaler spacing from a general rule. Use the form maker's tables.
- Ordering without a second method.
- Ordering the wrong strength. It comes from the drawings.
Calculate twice, with a different method the second time, for example area times thickness, then by sections. If the two answers disagree, find out why before you order. Write the order on the formwork drawing so the crew and the driver see the same number.
Hot, windy weather speeds up set time and drying. Cold weather slows strength gain and risks frozen concrete. Rain can dilute a fresh surface. Tell the plant about the conditions when you order, and have protection ready.
A footing is 30 m long, 450 mm wide and 250 mm deep. How much concrete, before waste?
You count the footing forms for 30 m, one course of 2.44 m boards, and get 13 boards. What did you forget?
The wall is 12 m long and 2.4 m high with ties at 600 mm by 600 mm. How many ties as a base figure?
Four solid concrete steps, 250 mm tread, 900 mm wide, 180 mm riser. Why is the volume not just one step times four?
A footing is 80 ft long, 16 in wide and 8 in deep. How many cubic yards before waste?
You work out 6.7 m³ two different ways and get 6.2 m³ the second time. What do you do?
- Must-haveAn 8 m (25 ft) tape and a chalk line
- Nice to haveA laser distance measurer, and a measuring wheel for long runs
- Must-haveA calculator or phone calculator, a scale rule, printed drawings, pencil, highlighter and an order sheet to write the m³ on
- Must-haveA builder's level or laser level to confirm depths before you order
- Must-haveForm material to count:
- Must-haveHard hat (CSA Z94.1), safety glasses (CSA Z94.3) and green-triangle boots (CSA Z195) for site visits
Home DIY
Laborer or helper
1st-year apprentice
Apprentice, 2nd to 4th year
Journeyperson
Lead hand or foreman
Contractor or business owner
Starting a new business
The arithmetic is the same across Canada. What changes is the code edition and the formwork safety rules. Concrete is specified to CSA A23.1/A23.2 everywhere.
- BC: BC Building Code 2024. Formwork safety is under WorkSafeBC OHS Regulation Part 20.
- AB: National Building Code 2023 Alberta Edition, in force 1 May 2024.
- SK: NBC 2020 with few amendments, from 1 Jan 2024.
- MB: Manitoba Building Code (M.R. 78/2023), from 1 Jan 2024.
- ON: 2024 Ontario Building Code (O. Reg. 163/24). Formwork duties are in O. Reg. 213/91.
- QC: Quebec Construction Code, Chapter I Building, in force 17 Apr 2025. Formwork rules are with CNESST.
- Elsewhere: ask the local authority and your provincial OHS regulator.
Formwork safety rules differ by province and we have not checked each one. Full comparison in Canada by Province.
- BC Building Code 2012, Division B, Section 9.3 (concrete strengths, air, water to cementing ratios; same article numbers as NBC)
- CSA A23.1/A23.2, Concrete materials and methods. Paid standard. Check the current edition.
- Volume, weight and conversion arithmetic is standard geometry, calculated and re-checked.
- Estimating allowances are typical trade practice, not code values.
Last reviewed October 2026. General information, not engineering or legal advice.