Development length is the length of bar that has to be embedded in concrete for the bar to reach its full yield strength without pulling out. Under CSA A23.3, a straight bar in tension needs at least 300 mm, a bar in compression at least 200 mm, and a standard hook at least 150 mm. For everyday Grade 400 bar in 30 MPa concrete, the simplified method gives roughly 300 mm for 10M, 430 mm for 15M and 520 mm for 20M bottom bars, with top bars, coated bars and weaker concrete needing more.
Development length vs embedment length
The two terms get used interchangeably on site, but they mean different things:
- Development length (ld) is what the code requires: the embedment a bar needs, past the point where it is fully stressed, to develop its yield strength through bond with the concrete.
- Embedment length is what the detail provides: the length of bar actually cast in beyond a critical section, such as the face of a support, the top of a footing or the end of a lap.
The rule is that embedment provided must be at least the development length required. A dowel that projects out of a footing, the bottom bars running into a support, a hooked bar anchored into a column: all of these are checked against development length.
Lap splices are built on the same number. A standard tension lap is 1.3 times the development length, which is why our lap splice chart uses the same assumptions as the tables below.
How CSA A23.3 calculates it
Clause 12 of A23.3 covers development and splices. For straight bars in tension there are two routes:
The simplified method (Clause 12.2.3):
ld = 0.45 × k1 × k2 × k3 × k4 × (fy ÷ √f'c) × db
where fy is the bar's specified yield strength in MPa, f'c is the concrete strength in MPa, and db is the bar diameter. The 0.45 coefficient applies where clear cover is at least one bar diameter, clear spacing is at least 1.4 bar diameters, and the member has minimum stirrups or ties through the development length (or is a slab or wall with clear spacing of at least 2 bar diameters). Other cases use 0.6, which makes the length one third longer.
The detailed method (Clause 12.2.2) uses the actual cover, spacing and transverse steel, and often gives shorter lengths where cover and confinement are generous. Engineers use it on congested or high-strength work.
In both methods, √f'c is capped at 8 MPa, and ld is never less than 300 mm.
The k factors adjust for site conditions:
| Factor | Condition | Value |
|---|---|---|
| k1, bar location | Horizontal bars with more than 300 mm of fresh concrete cast below them (top bars) | 1.3, otherwise 1.0 |
| k2, coating | Epoxy-coated bars | 1.2 or 1.5 depending on cover and spacing, 1.0 uncoated |
| k3, concrete density | Structural low-density or semi-low-density concrete | 1.3 or 1.2, 1.0 for normal density |
| k4, bar size | 20M and smaller | 0.8, otherwise 1.0 |
The product k1 × k2 need not be taken greater than 1.7.
Development length chart for 10M to 35M
Assumptions: CSA A23.3 simplified method with the 0.45 coefficient (conditions above must be met), Grade 400 bar (fy = 400 MPa), uncoated, normal-density concrete, ld not less than 300 mm, rounded up to the next 10 mm. "Top" means k1 = 1.3.
| Bar | 25 MPa bottom | 25 MPa top | 30 MPa bottom | 30 MPa top | 35 MPa bottom | 35 MPa top |
|---|---|---|---|---|---|---|
| 10M | 330 mm | 430 mm | 300 mm | 390 mm | 300 mm | 360 mm |
| 15M | 470 mm | 600 mm | 430 mm | 550 mm | 390 mm | 510 mm |
| 20M | 570 mm | 740 mm | 520 mm | 670 mm | 480 mm | 620 mm |
| 25M | 910 mm | 1,180 mm | 830 mm | 1,080 mm | 770 mm | 1,000 mm |
| 30M | 1,080 mm | 1,400 mm | 990 mm | 1,280 mm | 910 mm | 1,190 mm |
| 35M | 1,290 mm | 1,680 mm | 1,180 mm | 1,530 mm | 1,090 mm | 1,420 mm |
The jump from 20M to 25M is the k4 factor stepping from 0.8 to 1.0. In bar diameters, the bottom bar values work out to roughly 24 to 36 diameters, so the old "40 diameters" rule of thumb is not a reliable substitute.
What makes the number longer: epoxy-coated bar (multiply by 1.2 or 1.5, with k1 × k2 capped at 1.7), tight cover or spacing (0.6 coefficient), low-density concrete, and higher-strength bar. Because fy sits in the numerator, 500W bar needs a development length about 25% longer than 400W in the same concrete. See our rebar grades guide for what changes with grade.
Hooked bars in tension
Where there is not enough room for a straight bar, such as at the end of a beam framing into a column or at the edge of a footing, a standard hook shortens the anchorage. A23.3 Clause 12.5 sets a basic hook development length of:
lhb = 100 × db ÷ √f'c
and the final length is never less than 8 bar diameters or 150 mm. It is measured from the critical section to the outside of the hook.
| Bar | 25 MPa | 30 MPa | 35 MPa |
|---|---|---|---|
| 10M | 230 mm | 210 mm | 200 mm |
| 15M | 320 mm | 300 mm | 280 mm |
| 20M | 390 mm | 360 mm | 330 mm |
| 25M | 510 mm | 470 mm | 430 mm |
| 30M | 600 mm | 550 mm | 510 mm |
| 35M | 720 mm | 660 mm | 610 mm |
These are basic lengths for Grade 400 bar, rounded up to 10 mm. Modification factors then apply: fy ÷ 400 for other grades, 0.7 for 35M and smaller bars with at least 60 mm of side cover (and at least 50 mm of cover on the tail of a 90° hook), 1.2 for epoxy-coated bars, 1.3 for low-density concrete, and a reduction where more steel is provided than the analysis requires.
The hook itself (bend diameter and tail length) comes from the standard hook tables used by detailers, which our post on standard rebar hooks covers.
Bars in compression
Column bars and dowels that are only ever in compression develop more easily, because the bar bears on the concrete at its end as well as through bond. A23.3 Clause 12.3 sets the compression development length as the largest of 0.24 × fy × db ÷ √f'c, 0.044 × fy × db, and 200 mm:
| Bar | 25 MPa | 30 MPa and higher |
|---|---|---|
| 10M | 220 mm | 200 mm |
| 15M | 310 mm | 290 mm |
| 20M | 380 mm | 350 mm |
| 25M | 490 mm | 450 mm |
| 30M | 580 mm | 530 mm |
| 35M | 690 mm | 630 mm |
(Grade 400, rounded up to 10 mm.) From about 30 MPa up, the 0.044 fy db floor governs, so stronger concrete stops shortening the length. The length can be reduced where bars are enclosed by a spiral or closely spaced ties. Whether a bar is designed as a compression bar is the engineer's call. Most column dowels see tension under some load case, so do not assume the shorter compression value applies.
Dowels, "effective length" and calculators
Most searches for embedment length are really about dowels: how far a bar has to go into a footing, wall or existing slab. The development length above is the starting point, but dowels also depend on the depth of the element they go into, hooks at the bottom, and, for dowels drilled into existing concrete, the adhesive manufacturer's data rather than A23.3 bond rules. Our post on how long rebar dowels should be covers that in detail.
"Effective length" is sometimes used loosely to mean the embedded length that actually does work, measured past the critical section. It is not a term A23.3 uses for development; in concrete design, effective length usually refers to column buckling, which is a different topic.
Embedment length calculators and charts online are mostly built on US ACI 318 equations and US bar sizes (#4, #5). The numbers will not match A23.3 exactly, and the US bars are not the same size as Canadian bars. Our Canadian vs US rebar size guide shows how they compare.
Getting development lengths right on the bar list
Development lengths turn into real bar lengths on the bar list: how far bottom bars run past supports, how long dowels are, and where hooks are needed. If the drawings give development and lap lengths in a schedule, the detailer uses them directly. If they only say "develop per CSA A23.3", the detailer needs the concrete strength, bar grade, coating and cover to work them out, and should confirm the assumptions with the engineer.
We prepare bar lists and placing drawings from your structural drawings, with development, hook and lap lengths built into every bar mark, then cut, bend and tag the bar. Send us your drawings for a free take-off and quote, or see our rebar products for straight bar, dowels and fabricated shapes.
Frequently asked questions
What is the minimum development length for rebar?
Under CSA A23.3, a straight bar in tension needs a development length of at least 300 mm, whatever the calculation gives. A bar in compression needs at least 200 mm, and a standard hook at least 150 mm or 8 bar diameters.
What is the development length of 15M rebar?
For uncoated Grade 400 15M bars in 30 MPa concrete, the CSA A23.3 simplified method gives about 430 mm for bottom bars and 550 mm for top bars. Weaker concrete, epoxy coating, 500W bar or tight cover make it longer.
What is the development length of 20M rebar?
Using the same assumptions (Grade 400, uncoated, 30 MPa concrete, simplified method), 20M needs about 520 mm as a bottom bar and 670 mm as a top bar. Your drawings may show a different value if the engineer used the detailed method.
What is the difference between development length and embedment length?
Development length is the embedment a bar needs to reach its full yield strength. Embedment length is the length actually provided past a critical section. The embedment provided must be at least the development length required.
Is there a rebar embedment length calculator?
Online calculators exist, but most use US ACI 318 equations and US bar sizes. For Canadian work, the CSA A23.3 equations and the k factors for your job apply, and the governing length is the one on the structural drawings.
