To convert rebar spacing when you change bar size, keep the steel area per metre the same: new spacing = old spacing × (new bar area ÷ old bar area). Swapping 15M at 300 mm for 10M gives 300 × 100 ÷ 200 = 10M at 150 mm. Round the answer down to a spacing the crew can lay out, then check it against the spacing limits and get the swap approved on any designed work.
The formula is the easy part. This post walks through it with metric and imperial examples, then covers the checks that decide whether the new layout actually works.
The rebar spacing conversion formula
Rebar in a slab, wall or footing mat is specified as a bar size at a spacing, for example 15M @ 300. What the designer actually needs is a certain area of steel per metre of width:
Area per metre (mm²/m) = bar area (mm²) × 1,000 ÷ spacing (mm)
If you change the bar but want the same area per metre, the spacing has to change in proportion to the bar area:
New spacing = old spacing × new bar area ÷ old bar area
Canadian metric bars make this simple because their nominal areas are round numbers:
| Bar | Nominal area | Ratio to 10M |
|---|---|---|
| 10M | 100 mm² | 1 |
| 15M | 200 mm² | 2 |
| 20M | 300 mm² | 3 |
| 25M | 500 mm² | 5 |
| 30M | 700 mm² | 7 |
| 35M | 1,000 mm² | 10 |
So 15M has exactly twice the area of 10M, 20M three times, and 25M five times. Going from 15M to 10M halves the spacing; going from 10M to 20M triples it.
Rounding rule: always round the new spacing down to the next practical increment. Rounding down adds a little steel; rounding up removes steel the design asked for. Many details use 10 mm or 25 mm steps, but follow whatever increment the drawings use.
Worked metric conversions
These are common swaps on residential and light commercial work. The area per metre is the same on both sides of each row, before rounding.
| Specified | Area per metre | Converted to | Exact spacing | Practical spacing |
|---|---|---|---|---|
| 15M @ 300 | 667 mm²/m | 10M | 150 mm | 150 mm |
| 15M @ 400 | 500 mm²/m | 10M | 200 mm | 200 mm |
| 15M @ 250 | 800 mm²/m | 10M | 125 mm | 125 mm (or 120 mm) |
| 10M @ 200 | 500 mm²/m | 15M | 400 mm | 400 mm |
| 20M @ 300 | 1,000 mm²/m | 15M | 200 mm | 200 mm |
| 15M @ 200 | 1,000 mm²/m | 20M | 300 mm | 300 mm |
| 25M @ 300 | 1,667 mm²/m | 20M | 180 mm | 180 mm |
| 20M @ 250 | 1,200 mm²/m | 25M | 417 mm | 410 mm or 400 mm |
Our rebar area and spacing table lists the area per metre for every metric size at spacings from 100 to 500 mm, which is handy when you want to scan for options rather than calculate one.
Going up a size (10M to 15M, 15M to 20M) spreads the bars out, cuts the piece count and speeds up placing. Going down a size packs the bars closer, which is more pieces and more ties but usually better crack control. Both are legitimate; neither is free, as the checks below show.
Converting imperial callouts to metric bar
Details from US sources, older drawings or product literature often call for #4 @ 12 in or similar. The conversion is the same idea, with one extra step: turn the spacing into millimetres first (1 in = 25.4 mm).
US bar areas: #3 = 71 mm², #4 = 129 mm², #5 = 200 mm², #6 = 284 mm².
Example: #4 @ 12 in to 10M.
- Spacing in millimetres: 12 × 25.4 = 304.8 mm.
- Area per metre: 129 × 1,000 ÷ 304.8 = 423 mm²/m.
- 10M spacing: 100 × 1,000 ÷ 423 = 236 mm.
- Round down: 10M @ 230 mm (or 225 mm if the drawings work in 25 mm steps).
| US callout | Area per metre | 10M spacing | 15M spacing | 20M spacing |
|---|---|---|---|---|
| #3 @ 12 in | 233 mm²/m | 429 mm | 859 mm | n/a |
| #4 @ 16 in | 317 mm²/m | 315 mm | 630 mm | 945 mm |
| #4 @ 12 in | 423 mm²/m | 236 mm | 473 mm | 709 mm |
| #5 @ 16 in | 492 mm²/m | 203 mm | 406 mm | 610 mm |
| #5 @ 12 in | 656 mm²/m | 152 mm | 305 mm | 457 mm |
| #6 @ 12 in | 932 mm²/m | 107 mm | 215 mm | 322 mm |
Spacings in this table are exact, before rounding. Several of them, such as 15M at 859 mm, are mathematically correct and completely unusable, because they blow through the maximum spacing limit. That is the first of the checks.
Our Canadian vs US rebar sizes post covers bar-for-bar equivalents and grade differences if you are working through a full set of US details.
The checks after the math
Matching area per metre keeps the strength of the mat roughly the same. It does not automatically keep everything else the same. Before a converted layout goes on a bar list, check these:
1. Maximum spacing. Structural slabs designed to CSA A23.3 limit principal bars to the smaller of three times the slab thickness and 500 mm, and walls, beams and minimum slab steel have their own limits. A switch to a bigger bar often runs into this ceiling. Our maximum rebar spacing guide sets out the limits element by element.
2. Minimum clear spacing. Going to a smaller bar at close centres can squeeze the gap between bars. CSA A23.1 sets a minimum clear distance based on bar diameter, coarse aggregate size and an absolute minimum, so very tight layouts need checking, especially at laps where two bars sit side by side.
3. Cover and effective depth. A larger bar is thicker. If the chairs stay the same height, the centre of the steel moves and the effective depth changes slightly; if the top surface is fixed, the cover shrinks. Check the cover against the drawings.
4. Lap and development lengths. These depend on bar diameter. A 15M lap is longer than a 10M lap, so a bigger bar needs longer laps and longer hooks or embedment at the ends. If the existing details show lap lengths by bar size, use the row for the new bar. See the lap splice length chart.
5. Crack control. For the same area, smaller bars at closer spacing generally control crack width better than larger bars spread out. In beams and exposed elements, CSA A23.3 checks crack control directly using bar spacing and cover, so a swap to fewer, larger bars can fail a check that the original layout passed.
6. Two directions and two faces. A mat has bars both ways and a wall may have two faces. Convert each direction and each face separately. They do not have to use the same bar size, but each one has to keep its own area.
7. Approval. On any designed work, the conversion is a proposal. The engineer of record approves it, and the approved layout is what goes on the placing drawings. On a Part 9 house where the code or designer specifies the bar and spacing, ask the designer or building department before substituting.
What the change does to weight, pieces and labour
Because rebar mass is proportional to its area, a conversion that keeps the area per metre keeps the tonnage roughly the same. A 15M bar weighs twice as much per metre as a 10M bar (1.570 kg/m against 0.785 kg/m), and you use half as many. Rounding down adds a little, and longer laps on bigger bars add a little more.
What does change is the piece count:
- 15M @ 300 to 10M @ 150 doubles the number of bars. In a two-way mat where both directions change, it roughly quadruples the number of intersections. That is more handling and far more ties.
- 10M @ 200 to 15M @ 400 halves the bars, and in a two-way mat converted both ways it cuts intersections to about a quarter. Placing is faster, but each bar is heavier and the mat needs supports spaced for the new layout.
Chair spacing also follows bar size. Lighter bars sag between supports sooner, so a switch from 15M to 10M usually means more chairs.
To get the bar count across a width once you have the new spacing, use the method in our rebar spacing calculator post: clear width divided by spacing, plus one, rounded up.
Getting a converted layout onto the order
A spacing conversion is something we see often during take-offs, usually because a detail came from a US source or the contractor would rather place one bar size across the whole job. When you send drawings, tell us which substitution you are considering. We can price both layouts from the same drawings so the comparison is fair, and prepare the bar list once the engineer has approved the change.
Send your drawings for a free take-off and quote, or see the rebar we supply, from 10M to 55M, cut, bent and tagged to your bar list.
Frequently asked questions
How do you convert rebar spacing to a different bar size?
Multiply the old spacing by the new bar's area divided by the old bar's area. Going from 15M at 300 mm to 10M gives 300 × 100 ÷ 200 = 150 mm. Round down to a practical spacing, never up, and check it against the code limits.
What is 15M at 300 in 10M bar?
10M at 150 mm. A 15M bar has twice the area of a 10M bar, so halving the spacing keeps the same steel per metre (667 mm² per metre).
What metric spacing matches #4 at 12 inches?
A #4 bar at 12 in gives about 423 mm² of steel per metre. That works out to 10M at about 236 mm, so 10M at 230 mm, or 15M at about 473 mm, rounded down to 450 mm or 470 mm if the maximum spacing allows it.
Can I change rebar size without asking the engineer?
Not on designed work. A size swap changes lap and development lengths, cover, clear spacing and crack control, not just the area. Run the conversion, then submit it to the engineer of record for approval before ordering.
