◆ Calculators ◆ Rebar Calculator: How Many Bars, Splices and Pounds of Steel You Need
Rebar Calculator: How Many Bars, Splices and Pounds of Steel You Need
Work out how many rebar bars, splices and pounds of steel a slab or wall needs, with Ohio DOT's specification tables sourced.
12 in. is a common on-center spacing, not a value either source names — one spacing applies to both directions.
Ohio DOT Item 509, Table 509.05-1 — the eleven bar sizes this table covers, no interpolated size.
Neither source names a standard bar length — enter the length you're actually buying (e.g. 20 ft, 40 ft, or whatever your supplier stocks).
Ohio DOT lap lengths (Table 509.07-1), in inches — Uncoated / Epoxy Coated: #4 22/27 · #5 29/35 · #6 34/41 · #7 43/52 · #8 57/69 · #9 72/87 · #10 92/111 · #11 113/137. These are Ohio's values for bridge construction, not a national standard — enter your own value. No official lap length was found for #3, #14 or #18 in either source checked — enter a value based on your project's engineering requirements.
Neither source names a waste figure for rebar — 0% is a starting point, not a claim that no cutting or lap loss occurs.
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How the number is worked out
The steps from input to result
Reinforcing a slab or wall means a two-way grid of bars, not a single length — this calculator works out how many bars run in each direction from your area and bar spacing, then adds up what that grid needs in total length, weight and, separately, how many full-length stock bars you'd actually buy.
Ohio DOT, Construction and Material Specifications, Item 509 "Reinforcing Steel", Table 509.05-1 lists eleven bar sizes with their diameter, area and "Weight per Foot (lb.)" — this calculator uses exactly those eleven sizes and weights, no interpolated or invented size in between.
The same specification's Table 509.07-1 gives a "Lap Length (inches)" for splicing bars end to end, separately for uncoated and epoxy-coated bar, for sizes #4 through #11 — those are Ohio's own bridge-construction values, not a national standard, so this calculator asks for the lap length instead of assuming one.
A single reinforcing bar is one uncut piece, so the number of stock-length bars you have to buy is not simply the total length divided by the stock length — leftover pieces cut from one bar cannot be reused to complete a different bar, so each bar's own length, splices included, is rounded up to whole stock lengths first, and only then summed across both directions.
A waste allowance changes the weight and the bars-to-purchase figures differently: on the weight, it scales the already-spliced total length before that length becomes pounds, while on the purchase count it scales up the bar count in each direction before the stock-length rounding happens — never a percentage bolted onto an already-rounded bar count.
Texas DOT, 2024 Specifications, Item 440 "Reinforcement for Concrete", states that "the nominal size, area, and weight of reinforcing steel bars this Item covers are shown in Table 1." — an independent check against the same diameter and weight sequence as Ohio DOT's table, rather than resting on a single source.
The formula
Written out, so you can check it by hand
Bars per direction = round up(other side ÷ (spacing ÷ 12)) + 1, summed for the total bar count. Raw length = (bars spanning the length × length) + (bars spanning the width × width). For a bar longer than the stock length, stock lengths per bar = round up((span − lap ÷ 12) ÷ (stock length − lap ÷ 12)); splices = that minus 1, each adding one lap length (in feet). Steel weight needed = (raw length + splice allowance) × (1 + waste ÷ 100) × weight per foot of the chosen bar size. Bars to purchase per direction: if the span fits one stock length, round up((bar count × (1 + waste ÷ 100)) ÷ floor(stock length ÷ span)); otherwise (bar count × (1 + waste ÷ 100)) × stock lengths per bar — summed across both directions.
Worked example
Worked with the preset values
With a 25 ft × 10 ft area, 12 in. bar spacing and #4 bar (0.668 lb/ft): bars spanning the length (over the 10 ft width) = round up(10 ÷ 1) + 1 = 11; bars spanning the width (over the 25 ft length) = round up(25 ÷ 1) + 1 = 26 — 37 bars total. Raw length = 11 × 25 + 26 × 10 = 535 ft. Each of the 11 bars running the 25 ft length exceeds the 20 ft stock length, needing round up((25 − 22/12) ÷ (20 − 22/12)) = 2 stock lengths — 1 splice per bar, adding 11 × 22/12 ≈ 20.17 ft. The 26 bars running the 10 ft width fit in one stock length each, no splices. Total length with splices = 535 + 20.17 ≈ 555.17 ft, so the steel weight needed = 555.17 × 0.668 ≈ 370.85 lb. For bars to purchase: the 11 length-direction bars each need 2 stock lengths (11 × 2 = 22), and the 26 width-direction bars fit 2 per stock length (floor(20 ÷ 10) = 2, round up(26 ÷ 2) = 13) — 22 + 13 = 35 bars to purchase in total.
What this calculator does not do
Named, not left out
- 01No statement is made about concrete cover or edge distance — the governing standard, ACI 318, is paywalled and was not read for this calculator.
- 02The suggested lap lengths are Ohio's own values for bridge construction, not a national standard — the lap length field is yours to override.
- 03For bar sizes #3, #14 and #18, neither source checked names an official lap length — enter a value based on your project's engineering requirements.
- 04No bend shapes and no bend radii are covered — this calculator doesn't bend anything.
- 05No tie wire, chairs (rebar supports) or dobies are included.
- 06One reinforcement layer per run — for a second layer (e.g. top and bottom of a slab), run the calculator again.
- 07One shared bar spacing applies to both directions — there is no separate spacing field per direction.
- 08The bars-to-purchase figure assumes each bar is cut individually from one or more whole stock lengths — leftover pieces are never combined across different bars.
- 09One rectangular area per run — no diagonal reinforcement and no non-rectangular shapes.
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Updated: 9/8/2026