Rebar Development Length — Reference
Straight tension development, ACI 318-19 simplified method (§25.4.2.3). Grade 60, normalweight concrete, uncoated bars, favorable spacing/cover case. Minimum ld = 12 in.
Bottom Bars, Grade 60 — ld in inches (rounded up)
| Bar | db (in) | f′c = 3,000 | f′c = 4,000 | f′c = 5,000 |
|---|---|---|---|---|
| #3 | 0.375 | 17 | 15 | 13 |
| #4 | 0.500 | 22 | 19 | 17 |
| #5 | 0.625 | 28 | 24 | 22 |
| #6 | 0.750 | 33 | 29 | 26 |
| #7 | 0.875 | 48 | 42 | 38 |
| #8 | 1.000 | 55 | 48 | 43 |
| #9 | 1.128 | 62 | 54 | 48 |
| #10 | 1.270 | 70 | 61 | 54 |
| #11 | 1.410 | 78 | 67 | 60 |
In bar diameters: 43.8/37.9/33.9·db (≤ #6) and 54.8/47.4/42.4·db (≥ #7) at 3,000/4,000/5,000 psi respectively. Note the jump from #6 to #7 — the code's two-tier equation, not a typo.
Modifiers
| Condition | Factor |
|---|---|
| Top bar (> 12″ fresh concrete below), Ψt | × 1.3 |
| Epoxy-coated, cover < 3db or spacing < 6db, Ψe | × 1.5 |
| Epoxy-coated, other, Ψe | × 1.2 |
| Lightweight concrete, λ = 0.75 | × 1.33 |
| Simplified-method unfavorable spacing/cover case | × 1.5 |
| Class B tension lap splice | 1.3 × ld |
Ψt·Ψe need not exceed 1.7. Favorable case requires: clear spacing ≥ db, clear cover ≥ db, and code-minimum stirrups over ld — or clear spacing ≥ 2db with cover ≥ db.
Sources: ACI 318-19, §25.4.2 (development), §25.5 (splices), Table 25.4.2.5 (modification factors). Wight & MacGregor, Reinforced Concrete: Mechanics and Design. Values computed for fy = 60,000 psi; Grade 80 bars scale by 80/60 plus additional ACI provisions.
Worked example: one bar, four answers
#5 bar (db = 0.625 in), Grade 60, f′c = 4,000 psi, normalweight concrete
The same bar needs anywhere from 24 to 41 inches depending on where it sits and what it is coated with. Most detailing errors come from using the base value where a modifier applies. Top bars in a footing or slab more than 12 in thick and epoxy-coated bars in bridge decks and parking structures are the usual cases. Round up after applying the factors, not before.
Common mistakes
- Multiplying Ψt and Ψe without the 1.7 cap. Top epoxy-coated bars at 1.95 instead of 1.7 come out 15% longer than the code requires.
- Applying the ≤#6 equation to #7 and larger bars. The divisor changes from 25 to 20, which adds 25% length. That is why the table jumps between #6 and #7.
- Calling the splice length ld. A Class B lap is 1.3 ld. Class A (1.0 ld) is allowed only when the provided steel area is at least twice the required area and no more than half the bars are spliced in the lap length.
- Taking f′c above 10,000 psi. ACI caps √f′c at 100 psi for development, so higher-strength concrete does not shorten ld further.
When this card doesn’t apply
This card is the simplified ACI 318-19 §25.4.2.3 method for straight bars in tension. It does not cover compression development (§25.4.9), hooked bars (§25.4.3), headed bars (§25.4.4), bundled bars or welded wire reinforcement. Seismic detailing under Chapter 18 adds its own requirements. Bridge work under AASHTO LRFD uses a different development equation. The detailed §25.4.2.4 equation is permitted any time and is usually shorter with generous cover and stirrups.
Related cheat sheets and tools
Use with the RC beam flexure tool for φMn, the unit weights card for dead loads, and the beam deflection tool for serviceability.