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Rebar Development Length Calculator (ACI 318)

Estimate the tension development length needed for a rebar bar, using ACI 318's simplified case for adequately spaced/covered bars.

Computed in your browser — nothing you enter is uploaded. Figures are presented in United States units and terminology, and each formula’s basis is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Development length

23.7 in

Check your inputs

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
23.72 in
Formula constant (K) used
25

At the values currently entered, the development length works out to 23.7 in. The largest intermediate quantity is equivalent in mm, at 23.7 in — check that step first if the total looks off. Confidence is moderate: the method is sound, but real materials and site conditions vary. Figures are shown in United States units and terminology; switch the market above if you are building elsewhere.

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.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.

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.

Regulatory standards & verification citations

  • 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

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Frequently asked questions

What is development length?
It's the minimum embedded length a rebar needs on each side of a critical section to transfer its full tensile force into the surrounding concrete through bond, without slipping or pulling out.
Why does bar size change the formula's constant?
Larger bars (#7 and up) have proportionally less surface area per unit of cross-sectional area than smaller bars, so ACI 318 uses a smaller divisor (20 instead of 25) to account for their somewhat more efficient bond behavior relative to their size.
What if my spacing or cover doesn't meet the 'Case 1' minimums?
You need the full ACI 318 development length equation with additional modification factors for spacing/cover, which typically increases the required length — this simplified calculator only covers the adequately-spaced case.