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Rebar calculator: bar counts, weight, and tons
Enter slab or footing dimensions and bar spacing, get bar counts each way, total linear feet, pounds, and tons using ASTM standard bar weights. The formulas are printed below the results so you can check the math.
Formula: bars one way = floor(width in inches / spacing) + 1, bars other way = floor(length in inches / spacing) + 1. Total linear feet = (bars one way x length) + (bars other way x width). Pounds = total linear feet x weight per foot; tons = pounds / 2,000. Weights are ASTM A615 nominal values. This is the clean mat quantity; add your own lap allowance, dowels, and corner bars before ordering.
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How rebar is taken off
Rebar is quantified two ways depending on the member. Slabs, mats, and walls are taken off as a grid: you count the bars in each direction from the spacing, multiply each count by the bar length, and sum the linear feet. Individual members like beams, columns, and tie beams are taken off bar by bar from the reinforcement schedule, because their bars vary by length, bend, and position.
This calculator handles the grid case. Bars in one direction run the full length of the slab, and bars in the other direction run the full width. The count in a given direction is set by the perpendicular dimension and the spacing, plus one bar to close the edge.
Bars one way = floor(width in inches / spacing) + 1 Bars other way = floor(length in inches / spacing) + 1
Each one-way bar spans the length, and each other-way bar spans the width, so total linear feet is the sum of the two products. Multiply total linear feet by the weight per foot for the bar size to get pounds, then divide by 2,000 for tons.
Laps, dowels, and corners
The bar count above is the clean geometric quantity. Real mats carry more steel than the grid math shows. Bars come in stock lengths (commonly 20, 40, or 60 ft), so longer runs are spliced with a lap, and each lap adds overlapping bar that the spacing math does not capture. A typical lap allowance falls in the 5 to 10 percent range, with the exact figure driven by bar size, splice class, and the stock lengths you actually buy. Dowels at construction joints, corner bars, and extra top steel over supports are additional and are usually picked up as separate line items rather than folded into the mat percentage.
ASTM standard bar weights
US rebar weights are fixed by ASTM A615/A615M and are the same regardless of supplier. The values below are the published nominal weights per linear foot:
- #3: 0.375 in diameter, 0.376 lb per ft
- #4: 0.500 in diameter, 0.668 lb per ft
- #5: 0.625 in diameter, 1.043 lb per ft
- #6: 0.750 in diameter, 1.502 lb per ft
- #7: 0.875 in diameter, 2.044 lb per ft
- #8: 1.000 in diameter, 2.670 lb per ft
- #9: 1.128 in diameter, 3.400 lb per ft
- #10: 1.270 in diameter, 4.303 lb per ft
- #11: 1.410 in diameter, 5.313 lb per ft
Worked example
Take a 20 ft by 20 ft slab with #5 bars at 12 in on center each way, matching the calculator defaults. Bars one way = floor(240 / 12) + 1 = 21, and bars other way = floor(240 / 12) + 1 = 21. The one-way bars each span 20 ft (21 x 20 = 420 lf) and the other-way bars each span 20 ft (21 x 20 = 420 lf), for 840 total linear feet. At 1.043 lb per ft for #5, that is 840 x 1.043 = 876 lb, or 0.44 tons before any lap allowance. Add your typical lap percentage on top before you order.
From bars to a priced line
The bar count and tonnage are the quantity half of the estimate. The priced line multiplies the tons by an all-in unit cost covering material, fabrication, delivery, and placement labor. That is the step Ruh automates: it reads the slab and footing dimensions and reinforcing notes off your drawings, computes the mats with your lap allowances, and prices them against your own price book, with your estimator reviewing the result. For the units involved see the linear foot glossary page, and for the broader workflow see quantity takeoff.
Rebar calculator FAQs
How much does rebar weigh per foot for a #5 bar?+
A #5 bar weighs 1.043 lb per linear foot, which is the ASTM A615 nominal weight. The bar designation roughly tracks eighths of an inch in diameter, so #5 is about 5/8 in, and the weight scales with the square of the diameter.
How do I calculate rebar for a slab?+
Count bars each way as floor(perpendicular dimension in inches divided by spacing) plus one, multiply each count by the span it covers to get linear feet, add the two totals, then multiply by the weight per foot for the bar size. Divide by 2,000 for tons and add a lap allowance before ordering.
What lap length allowance should I add?+
A typical lap allowance falls in the 5 to 10 percent range on the clean bar quantity, with the exact figure driven by bar size, splice class, and the stock lengths you buy. Dowels, corner bars, and extra top steel are usually picked up as separate line items rather than folded into the percentage.
Ruh does this for the whole drawing set.
AI takeoff reads your plans, computes every mat and member, and prices it on your price book.
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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.