A closed rectangular stirrup's cut length is the sum of its out-to-out sides plus the hook allowance at each end: L = 2 × (A + B) + 2 × hook. For a standard 135° hook, the RSIC hook table gives about 100 mm on 10M and 140 mm on 15M. Get A and B from the beam size minus twice the cover, count stirrups from the spacing, and multiply by the bar mass to get the weight. That gives a slightly generous length that suits estimating; the shop's detailing software computes the exact bent length for fabrication.
This post works through the formulas for each common stirrup and tie shape, then a full beam example with two spacing zones. For hook dimensions and stirrup sizes in more depth, see stirrup sizes and hook lengths.
Step 1: Get the out-to-out dimensions
Stirrups are dimensioned out-to-out: from the outside of one leg to the outside of the other. Start from the concrete:
Out-to-out width = beam width minus 2 × cover Out-to-out height = beam depth minus 2 × cover
The cover here is the cover to the stirrup, since the stirrup is the outermost bar. Where the drawings don't give a stirrup size, the RSIC Manual of Standard Practice details stirrups 80 mm less than the width and depth of the beam, which is the same as 40 mm cover on every face. If your cover is different, for example a beam cast against earth, use the real cover. Our post on concrete cover under CSA A23.1 has the values.
Column ties work the same way. RSIC details them 80 mm less than the column's outside dimensions unless specified otherwise.
Step 2: Pick the hook allowance
The hook allowance is the extra bar each hook needs beyond the out-to-out dimension. These are the RSIC stirrup and tie hook values (Table 5D):
| Bar size | 90° hook | 135° standard hook | 135° seismic hook |
|---|---|---|---|
| 10M | 100 mm | 100 mm | 140 mm |
| 15M | 140 mm | 140 mm | 150 mm |
| 20M and larger | Use the 180° hook values from the standard hook table |
The same table sets the finished bend diameter at 45 mm for 10M and 65 mm for 15M. Seismic hooks have longer tail extensions so they stay anchored in the core if the cover spalls. Use them where the drawings call for seismic detailing. For 20M and larger, the values are in our rebar hook length table.
Step 3: Apply the formula for the shape
Using A for width and B for height, both out-to-out:
| Shape | RSIC type | Cut length |
|---|---|---|
| Closed stirrup or tie, hooks at one corner | T1 (135°) or T2 (90°) | 2 × (A + B) + 2 × hook |
| Open U stirrup, hooked legs | S-types | 2 × B + A + 2 × hook |
| U stirrup without hooks (hairpin or plain U) | Per drawing | 2 × B + A |
| Two-piece closed stirrup: U plus cap tie | U plus T5 or T9 | U: 2 × B + A + hooks; cap: A + 2 hooks |
| Crosstie, 135° one end and 90° the other | T9 | A + 135° hook + 90° hook |
| Circular tie closed with 135° hooks | T3A | π × (D minus bar diameter) + overlap + 2 × hook |
| Circular hoop closed by a lap | T3 | π × (D minus bar diameter) + lap |
A few notes on the shapes:
- Two-piece stirrups are used where longitudinal bars have to be threaded through. RSIC warns against springing closed stirrups open to drop bars in, because it can crack them.
- Multi-leg stirrups in wide beams are usually an outer closed stirrup plus either crossties or a second, narrower closed stirrup. Calculate each piece separately.
- Circular ties: D is the outside diameter. RSIC sets the T3A overlap at 150 mm minimum, or enough to hook around two adjacent vertical bars, whichever is greater. The T3 lap is a Class B tension splice, which comes from the engineer.
Why the simple formula runs a little long
Adding out-to-out sides counts each corner as if the bar turned a sharp right angle at the outside face. In reality the bar curves around the pin, so the bent bar is a little shorter than the sum. Many international bar bending schedule methods subtract a "bend deduction" for each bend to correct for this.
For estimating, the simple sum is the safer choice: it overstates slightly, which covers waste. For fabrication, Canadian shops work from the out-to-out dimensions and hook values on the bar list, and their software produces the exact cut length. The fabricating tolerance for stirrups and ties under RSIC practice is plus or minus 13 mm, and RSIC asks for bar lengths to the nearest 20 mm. Our post on the rebar cutting length formula explains bend deductions in detail.
Step 4: Count the stirrups
The count comes from the spacing on the drawings:
Number of spaces = length covered ÷ spacing, rounded up Number of stirrups = number of spaces + 1
Round the spaces up, never down, so the actual spacing never exceeds what the drawing allows. RSIC practice places the first and last stirrup within 50 mm of the face of the support unless the drawings say otherwise. Many beams have closer spacing near the supports and wider spacing in the middle; count each zone separately and don't double count the stirrup at the boundary.
Worked example: a beam with two spacing zones
A 350 mm wide by 500 mm deep beam, 5,400 mm clear span, closed 10M stirrups with standard 135° hooks. Spacing: 100 mm for the first metre at each end, 200 mm in the middle. No stirrup size on the drawings, so use the RSIC 80 mm rule.
Dimensions. 350 minus 80 = 270 mm wide; 500 minus 80 = 420 mm high.
Cut length. 2 × (270 + 420) + 2 × 100 = 1,380 + 200 = 1,580 mm.
End zones. The first stirrup sits 50 mm from the support and stirrups continue at 100 mm to 1,050 mm from the face: (1,050 minus 50) ÷ 100 = 10 spaces, so 11 stirrups. Two ends: 22 stirrups.
Middle zone. From 1,050 mm to 4,350 mm is 3,300 mm. 3,300 ÷ 200 = 16.5, rounded up to 17 spaces. The stirrups at each end of this zone are already counted, so add 17 minus 1 = 16 stirrups.
Total. 22 + 16 = 38 stirrups.
Length and weight. 38 × 1.580 m = 60.0 m. At 0.785 kg/m for 10M, that is about 47 kg.
Two quick checks on the same beam, if the design called for them:
- A 10M crosstie across the 270 mm width with a 135° hook and a 90° hook: 270 + 100 + 100 = 470 mm.
- Switching to 15M stirrups with the same dimensions: 1,380 + 2 × 140 = 1,660 mm each, at 1.570 kg/m, or about 99 kg for 38 pieces. Bar size has a big effect on weight.
Mistakes that throw off stirrup quantities
- Using the beam size instead of out-to-out. That makes every stirrup 80 mm too big each way, and they won't fit in the forms.
- Forgetting the hooks. Two 135° hooks on 15M add 280 mm per stirrup.
- Rounding spaces down. The spacing ends up wider than the drawing allows.
- Missing spacing zones. Tighter spacing near supports can nearly double the count in short beams.
- Ignoring seismic hooks where they are specified. They are longer.
- Counting stirrups through column zones where the drawings stop them, or missing stirrups that continue through.
If you are building the full list for a job, the same counts go into a bar list; see how to make a rebar cutting list.
Get stirrups made to your schedule
We detail stirrups and ties from your structural drawings, then cut, bend, tag and bundle them by mark, along with the rest of your rebar from 10M to 55M. Send a stirrup schedule or the beam drawings and we will prepare a free take-off and quote.
Request a quote, or see our services for detailing and cut and bend.
Frequently asked questions
What is the formula for stirrup cut length?
For a closed rectangular stirrup: cut length = 2 × (width + height) + 2 × hook allowance, using out-to-out dimensions. In RSIC practice the hook allowance is about 100 mm for a standard 135° hook on 10M and 140 mm on 15M. Adding out-to-out sides slightly overstates the true bent length, which is fine for estimating.
How do you work out stirrup dimensions from the beam size?
Out-to-out stirrup size = beam dimension minus twice the cover to the stirrup. Where the drawings don't say otherwise, RSIC practice details stirrups 80 mm smaller than the beam in width and depth.
How many stirrups are in a beam?
Number of spaces = (length to be covered ÷ spacing), rounded up; number of stirrups = spaces + 1. Measure from the first stirrup to the last, which RSIC practice places within 50 mm of each support face unless the drawings say otherwise. Count each spacing zone separately.
How do you calculate the weight of stirrups?
Multiply the cut length in metres by the number of stirrups and by the bar mass: 0.785 kg/m for 10M and 1.570 kg/m for 15M.
