Free rebar calculator for concrete slabs: count rebar bars, total linear feet, weight in pounds, and estimated cost from your slab size, spacing, and bar size.
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Enter slab length and width in feet, pick the rebar bar size, and set the on-center spacing in inches. The fence post rule counts bars per direction: the dimension in inches divided by the spacing, rounded up, plus one. Each count times its span gives total linear feet of steel. That footage times the bar size's standard weight per foot gives total weight in pounds; price per pound plus your waste percentage gives the estimated material cost. Planning estimates, not professional advice: confirm bar size, spacing, and cover against your drawings and local code before pouring.
This rebar calculator figures out exactly how much steel your concrete slab needs. Enter the slab length and width, pick a bar size, and set the on-center spacing. The tool counts the bars in each direction using the fence post rule (divide the dimension by the spacing and add one), then totals the linear feet of rebar across the whole grid. Multiply that footage by the weight of the selected bar size to get the total weight in pounds, and the price-per-pound estimate gives you a realistic material cost for ordering. It is a fast, honest starting point for a materials list before you place an order with a supplier.
Rebar is sold by number sizes that correspond to diameter. Common slab sizes are #3 (3/8 inch, 0.376 lb/ft), #4 (1/2 inch, 0.668 lb/ft), #5 (5/8 inch, 1.043 lb/ft), and #6 (3/4 inch, 1.502 lb/ft). The size you need depends on the slab thickness, the loads the slab will carry, and local code requirements, so confirm with your plans or engineer before buying. Heavier bars like #5 and #6 add a lot of weight per foot, which raises both the handling effort and the cost. The calculator uses standard ASTM unit weights for each bar size, so the weight figure you get is consistent with what suppliers and yard scales use.
Walk through a real example: a 20 ft by 12 ft garage slab with #4 rebar spaced 12 inches apart. Bars along the length: 12 × 12 / 12 + 1 = 13 bars, each 20 ft long. Bars along the width: 20 × 12 / 12 + 1 = 21 bars, each 12 ft long. That is 34 bars total and 13 × 20 + 21 × 12 = 512 linear feet of steel. At 0.668 lb/ft for #4 bar, the weight is 512 × 0.668 = 342.02 lbs. At $0.75 per pound with 10% waste, the estimated cost is about $282.16. Notice how both directions are counted separately, since a bar running the length of the slab is spaced across its width, and vice versa.
Spacing is the biggest lever on your rebar quantity. For typical residential slabs, patios, and driveways, 12 to 18 inches on center in both directions is common, usually with #3 or #4 bar. Going from 18-inch to 12-inch spacing nearly doubles the number of bars, so do not guess this value. Tighter spacing carries more load but costs more in steel and labor. Driveways under vehicle loads often use #4 at 12 inches, while light patios can be fine with #4 at 18 inches. Always check your drawings and local building code for the required spacing before you order, since the calculator estimates quantity from the spacing you enter rather than choosing it for you.
Rebar is priced by weight, so the total pounds drive the bill. Enter the supplier's price per pound and a waste percentage to cover lap splices, offcuts, and field mistakes. A 10% waste factor suits simple rectangular slabs; bump it to 15% or more on jobs with many splices, heavier bar sizes (whose lap lengths grow with diameter), or irregular layouts. Remember that labor, delivery, tie wire, and chairs are not included in this estimate. A rebar cost estimate of a few hundred dollars can still grow once the crew, ties, and delivery charges are added, so use this figure as a materials budget baseline, not the final project price.
Three mistakes cost people the most. First, counting bars in only one direction, which leaves you about half the steel you need. Second, forgetting the plus one in the bar count (the fence post rule), which shortchanges every run by one bar. Third, entering spacing in feet when the calculator expects inches, or the reverse, which produces wildly wrong bar counts. Also remember that a flat waste percentage does not fully capture lap splice length, which grows with bar diameter, so heavy-bar jobs may need a larger waste factor. Double-check your units before hitting calculate, and round up when ordering since suppliers sell whole bars.
This steel reinforcement calculator follows the standard layout rule used for concrete reinforcement: bars are spaced evenly on a grid in both directions, and the total rebar quantity is the count in each direction plus a waste allowance. Rebar spacing for a concrete driveway is commonly 18 to 24 inches for #4 bars, while a rebar calculator for a garage slab usually assumes 12 to 18 inch spacing because of vehicle loads. Bars are normally sold as 20 ft sticks, so this page also tells you how many 20 ft rebar sticks to order: divide the total linear feet by 20 and round up. As a quick per square foot estimator, a 12 inch grid of #4 bars works out to roughly 0.67 lb of rebar per square foot, and you can compare that rule of thumb against the calculator result.
Count the bars in each direction separately using the fence post rule: divide the dimension in inches by the spacing and add one. Bars along the length = (width × 12 / spacing) + 1, and bars along the width = (length × 12 / spacing) + 1. Add the two together for the total bar count. For example, a 20 ft by 12 ft slab at 12-inch spacing needs 13 + 21 = 34 bars.
Bars sit at both edges of the grid plus at every spacing interval between, so the number of bars is always one more than the number of gaps. This is the classic fence post rule: dividing the run by the spacing gives you the gaps, and adding one gives you the bars. Skip the plus one and every run comes up one bar short.
Common residential spacing is 12 to 18 inches on center in both directions for a 4-inch slab using #3 or #4 rebar. Driveways under vehicles often use #4 at 12 inches, while light patios can use #4 at 18 inches. Tighter spacing increases steel and load capacity. Always follow your local code or engineering drawings, since spacing is a design decision, not a guess.
Each lengthwise bar runs the full slab length and each widthwise bar runs the full slab width. Multiply the bar count in each direction by its span, then add the two figures. This calculator shows that total, plus a weight estimate from the bar size and an optional cost estimate using your waste percentage.
You tie the bars where they cross. Multiply the lengthwise bar count by the widthwise bar count to get the number of intersections, which is roughly your tie-wire point count. Many crews plan for about 1.5 ties per crossing to cover re-ties during the pour.
Total weight = total linear feet × the bar's weight per foot. #4 rebar weighs 0.668 lb/ft, so 500 linear feet of #4 bar weighs about 334 lbs. Estimated cost = weight × price per pound × (1 + waste/100). This gives a materials-only estimate; labor, delivery, and chairs are not included.
For a 20 ft by 20 ft slab with #4 rebar at 12-inch spacing in both directions and 10% waste, the math gives 21 + 21 = 42 bars, 840 linear feet of steel, about 561 lbs (0.28 tons). Enter your exact dimensions in the calculator above for your precise figure.
A waste factor covers lap splices (bars lapped where runs exceed stock length), offcuts at the edges, and field mistakes. Simple rectangular slabs work with 5-10%; jobs with many splices or heavier bars may need 15% or more. Note that a flat waste percentage does not fully capture lap splice length, which grows with bar diameter, so bump the factor up on heavy-bar work.