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The rebar bar size being developed.
ACI's constant K is 25 for #6 and smaller and 20 for #7 and larger. K divides, so the larger bars' smaller K makes the development length longer, not shorter — exactly 1.25 times as much per diameter of the bar's own thickness, all else equal.
The rebar's specified yield strength.
60,000 psi (Grade 60) is standard for US construction rebar.
The concrete's specified compressive strength.
ACI 318 caps sqrt(f'c) at a value corresponding to 100 psi (f'c=10,000psi) in this formula for very high-strength mixes.
A bar with a lot of fresh concrete under it develops less bond.
Bleed water and settlement collect under a horizontal bar while the concrete below it consolidates, leaving a weaker bond on its underside. ACI 318 answers that with a 30% longer development length for any bar with more than 12 in of concrete cast below it, which on a deep beam or a wall means the top mat.
Epoxy is a bond breaker as well as a corrosion barrier.
The coating that stops the bar rusting also stops it gripping, so an epoxy-coated bar needs a longer embedment. How much longer depends on cover and spacing, because a well-covered bar can still mobilise the concrete around it. ACI caps the product of this and the casting-position factor at 1.7 however the two are chosen, and this page applies that cap.
1.0 for normal-weight concrete; 0.75 for all-lightweight concrete.
Lightweight concrete bonds less effectively to rebar, requiring a longer development length.
Development length
23.7 in
This is ACI 318's simplified Case 1 equation, valid only when clear spacing and cover meet the code's minimum thresholds and minimum ties/stirrups are provided — tighter spacing or less cover requires the full (longer) development length equation with additional modification factors.
- Equivalent in mm
- 602.41 mm
- Formula constant (K) used
- 25
They open the calculator with your figures already in it
Rebar Development Length Calculator (ACI 318): 23.72 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- ACI 318 simplified development length equation for Case 1 (clear spacing >= db and clear cover >= db, or clear spacing >= 2db, with minimum code-specified ties/stirrups): ld = [fy x psi_t x psi_e / (K x lambda x sqrt(f'c))] x db, where K = 25 for #6 bars and smaller, K = 20 for #7 bars and larger
Inputs used
- Rebar Bar Size
- #5 (5/8 in)
- Steel Yield Strength (fy)
- 60000 psi
- Concrete Strength (f'c)
- 4000 psi
- Casting Position Factor (psi_t)
- Other than a top bar (psi_t = 1.0)
- Epoxy Coating Factor (psi_e)
- Uncoated or zinc-coated (psi_e = 1.0)
- Lightweight Concrete Factor (λ)
- 1
Intermediate steps
- Equivalent in mm
- 602.41 mm
- Formula constant (K) used
- 25
Confidence note: This is ACI 318's simplified Case 1 equation, valid only when clear spacing and cover meet the code's minimum thresholds and minimum ties/stirrups are provided — tighter spacing or less cover requires the full (longer) development length equation with additional modification factors.
What this calculation does not cover
- Case 1 spacing and cover are assumed, never checked. There is no input for clear spacing, clear cover or transverse reinforcement, so the page cannot tell whether your bar actually qualifies for the simplified equation — you confirm that off the drawing. A bar that falls outside Case 1 needs ACI's full development length equation, which returns a longer number than this one.
- This covers straight deformed bars in tension only. Standard hooks, headed bars and bars developed in compression follow different ACI expressions and are shorter; a lap splice is longer than the development length returned here. Do not carry this figure into any of those details.
- Only US bar designations #3 through #11 are offered, and only ACI rules are applied. #14 and #18 bars are not in the list, and neither are soft-metric or European bar sizes — the soft-metric figure is an ACI answer restated, not a Eurocode 2 or CSA A23.3 anchorage length.
- Seismic detailing is outside this calculation. ACI 318's provisions for special moment frames and special structural walls carry their own anchorage and splice requirements in the higher seismic design categories, and none of them are in this equation.
- This returns a required length, not a design. It does not check that the column, footing or wall you are anchoring into is deep enough to provide that embedment, does not locate the critical section the length is measured from, and does not replace the engineer of record's stamped detail.
Add the equipment this sizes
This result is a specification — 23.7 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- ACI 318 simplified development length equation for Case 1 (clear spacing >= db and clear cover >= db, or clear spacing >= 2db, with minimum code-specified ties/stirrups): ld = [fy x psi_t x psi_e / (K x lambda x sqrt(f'c))] x db, where K = 25 for #6 bars and smaller, K = 20 for #7 bars and larger
Which documents these citations point at
Standards referenced: ACI 318 (American Concrete Institute, United States).
Cite this page
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Tools and safety for this job
To fix reinforcement. Generic types, no brands, no prices.
Protection this work requires
- Saws, grinders and breakers run above 85 dB, where hearing damage accumulates and does not recover: defenders or plugs for every cut, not just the long ones.
- Breakers and grinders cause permanent nerve damage: limit continuous trigger time, keep hands warm, and stop if fingers tingle or blanch.
- Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
- Nailing, chiselling and cutting all throw fragments: glasses to EN 166 or ANSI Z87.1, and goggles rather than glasses overhead.
Show the 4 tools this job needsHide tools
Essential
Tape measure
Recommended
Rebar cutter and bender
Rebar tying tool
Optional
Angle grinder
Also needed as materials: cutting and diamond discs, tie wire.
what each concrete tool is for, and the spec that decides which to buy where one does.