The rebar cutting length formula is: cutting length = sum of the out-to-out leg dimensions, minus a bend deduction for each bend, plus the hook allowance for any standard 135° or 180° hook. The deduction exists because a bar bent around a pin takes a shorter path along its centreline than its outside dimensions suggest. For a 90° bend on Canadian Grade 400W bar, that deduction works out to roughly 27 mm for 10M, 39 mm for 15M and 45 mm for 20M, using the minimum bend diameters.
This post explains where the formula comes from, gives calculated deductions for 10M to 55M, and works through examples. It is for estimators, detailers and anyone checking a bar list. You do not need it to order rebar: you give the shop the out-to-out leg dimensions and the bend type, and the cut length is worked out for you.
Why the cut length is shorter than the legs
On a Canadian bar list, leg dimensions are out-to-out: measured to the outside face of the bar at each bend. That is the practice in the RSIC Manual of Standard Practice, with one exception: the A or G dimension on standard 135° and 180° hooks, which follows the hook tables instead.
Out-to-out dimensions are what the crew needs, because they describe how the bar fits inside the formwork. But the bar does not have a sharp outside corner. It wraps around a bending pin, and the length of steel that goes round the bend is measured along the bar's centreline, which cuts the corner. Add up the out-to-out legs and you count that corner twice, so the straight bar the shop shears is a little shorter than the total of the legs.
The terms vary: bend deduction, bending allowance, bend length or bend correction. They all describe the same difference.
The bend deduction formula
For a bend of angle θ with leg dimensions measured to the outside of the bar:
Bend deduction = 2 × (r + db) × tan(θ ÷ 2) − θ × (r + db ÷ 2)
where:
- db is the bar diameter
- r is the inside bend radius, half the finished bend diameter D
- θ is the bend angle in radians (90° = 1.571, 45° = 0.785)
The first term is how much of the bar the two out-to-out dimensions assume lies in the corner. The second term is how much steel is actually needed to go round the bend along the centreline. The difference is the deduction.
For the most common case, a 90° bend, tan(45°) = 1 and the formula simplifies to:
90° deduction = 2 × (r + db) − 1.571 × (r + db ÷ 2)
The inside bend diameter comes from CSA A23.1, tabulated for Canadian bar sizes in the RSIC manual and in our rebar bending chart. Shops bend to these diameters as standard, and RSIC practice does not allow reducing them by more than 10% without the engineer's permission, so the minimum diameter is the usual starting point.
Calculated bend deductions for 10M to 55M
The table below applies the formula to Grade 400W or 500W bar at the minimum finished bend diameters for main bars. These are calculated geometric values, rounded to the nearest millimetre, not a published standard table.
| Bar size | db (mm) | D (mm) | 90° bend deduction | 45° bend deduction |
|---|---|---|---|---|
| 10M | 11.3 | 60 | 27 mm | 6 mm |
| 15M | 16.0 | 90 | 39 mm | 9 mm |
| 20M | 19.5 | 100 | 45 mm | 11 mm |
| 25M | 25.2 | 150 | 63 mm | 14 mm |
| 30M | 29.9 | 200 | 79 mm | 17 mm |
| 35M | 35.7 | 250 | 97 mm | 21 mm |
| 45M | 43.7 | 400 | 139 mm | 28 mm |
| 55M | 56.4 | 550 | 187 mm | 36 mm |
A few things to notice:
- The 90° deduction is about 2.3 to 2.7 bar diameters for 10M to 35M, and over 3 bar diameters for 45M and 55M, because those sizes are bent around much larger pins.
- R grades, coated bars and stirrups use different bend diameters, so their deductions differ. Epoxy-coated and galvanized bar are bent around larger pins; stirrups and ties around smaller ones.
- Real bending is not perfect geometry. Springback, pin wear and how each machine is set up mean shops calibrate their own allowances. Treat the table as the logic behind the number, not as a replacement for the fabricator's setup.
You will see rules of thumb quoted online, such as one bar diameter for a 45° bend and two for a 90° bend. They are rough approximations based on tighter bends than Canadian minimums, and they do not account for bar size. For metric bar bent to CSA minimums, the calculated values above are a better guide.
Worked examples
Example 1: 15M L-bar, 1,500 mm by 600 mm out-to-out, one 90° bend.
- Sum of legs: 1,500 + 600 = 2,100 mm
- One 90° deduction for 15M: 39 mm
- Cutting length: 2,100 − 39 = 2,061 mm
RSIC practice gives bar lengths to the nearest 20 mm unless a note flags a critical dimension, so on a bar list this reads as about 2,060 mm.
Example 2: 15M U-bar, legs 600, 1,200 and 600 mm out-to-out, two 90° bends.
- Sum of legs: 600 + 1,200 + 600 = 2,400 mm
- Two 90° deductions: 2 × 39 = 78 mm
- Cutting length: 2,400 − 78 = 2,322 mm, about 2,320 mm
Example 3: 20M bar, 3,000 mm straight with a standard 180° hook at one end.
Hooks work differently. The 180° hook dimension on a bar list is not out-to-out, so you add the standard hook allowance instead of deducting. For 20M Grade 400W bar the RSIC 180° hook allowance (G) is 200 mm:
- Cutting length: 3,000 + 200 = 3,200 mm
The hook allowances for every size, and the formulas behind them, are in our rebar hook length table.
Does the deduction matter?
For estimating tonnage, usually not much. RSIC fabricating tolerances allow plus or minus 25 mm on the overall length of a bent bar, and plus or minus 13 mm on stirrups and ties. A 39 mm deduction on a 15M L-bar is about 60 grams of steel. Many estimators simply add up the out-to-out legs, knowing the result is slightly high; our rebar estimating guide takes that approach.
It matters more when:
- Bars have many bends. A stirrup or a bar with several offsets adds the error up at every corner.
- Large bars are bent. A 35M bar loses close to 100 mm per 90° bend.
- You cut and bend on site. If you shear a bar to the sum of the legs and then bend it, every leg ends up long, and the bar may not fit the forms or keep its cover.
- Fit is tight. Closed ties and stirrups in a congested column or beam must close exactly.
For stirrups specifically, see our guide on how to calculate stirrup cut length, which combines the small stirrup bend diameters with 135° hooks.
Getting the length right on your order
You do not have to calculate cut lengths to order bent rebar. On a bar list, give:
- The bar mark, quantity, size and grade
- The bend type from the RSIC typical bar bends
- The out-to-out leg dimensions (A, B, C and so on)
- Any critical dimension that needs tighter than standard tolerance, flagged with a note
The detailer works out the total length, and the shop shears and bends to it. Our guide on how to order cut and bend rebar explains the list format.
Send us your bar list
Procut details, cuts and bends 10M to 55M rebar in our Erin shop, with every bundle tagged by mark so bars go in without sorting on site. Send us your structural drawings or your own bar list and we will prepare a free take-off and quote.
Request a quote, or see our rebar detailing and cut and bend services.
Frequently asked questions
What is the formula for rebar cutting length?
Cutting length equals the sum of the out-to-out leg dimensions, minus a bend deduction for each bend, plus the hook allowance for any standard 135 or 180 degree hook. The deduction exists because the bar's centreline takes a shorter path around a bend than the outside dimensions suggest.
What is the bend deduction for a 90 degree bend in rebar?
It depends on the bar size and bend diameter. Working from the minimum bend diameters for Grade 400W bar, the geometric deduction is roughly 27 mm for 10M, 39 mm for 15M, 45 mm for 20M and 63 mm for 25M per 90 degree bend.
Do I need to calculate cutting length when ordering rebar?
No. On a Canadian bar list you give the out-to-out leg dimensions and the bend type, and the detailer and fabricator work out the cut length. Calculating it yourself is useful for estimating weight and checking a bar list, not for ordering.
Is the 2d rule for a 90 degree bend correct?
It is a rough rule of thumb, not a Canadian standard value. With the minimum bend diameters used for metric bar in Canada, the geometric deduction for a 90 degree bend works out to about 2.3 to 2.7 bar diameters for 10M to 35M, and more for 45M and 55M.
