BYTETOOLS

Rebar Calculator

Calculate slab rebar: bars each way from your spacing, lap splices at 40 bar diameters, total linear feet, tie intersections and weight from the ASTM bar table.

21
Bars running lengthways
31
Bars running crossways
52
Total bars in the mat
1,224.0
Bar length before laps (ft)
21
Lap splices
1.67
Length of one lap (ft)
1,322.0
Total rebar (ft)
67
Stock bars to buy (20.0 ft)
883.1
Weight (lb)
400.6
Weight (kg)
651
Tie intersections
759.5
Tie wire (ft)
$794.76
Estimated rebar cost

Bars each way = (clear span ÷ spacing) + 1, where the clear span is the slab dimension less the concrete cover at both edges. Weights use the ASTM A615 nominal masses — 0.376 lb/ft for a #3 up to 2.670 lb/ft for a #8 — and the metric equivalents from 0.006165 × d², so a 25 mm bar is 3.853 kg/m. A 20 × 20 ft slab with #4 at 12 in each way and 3 in cover comes to 21 bars per direction, about 820 linear feet and roughly 570 lb of steel once a 5% allowance is added. Bar size, spacing, cover and lap length are engineering decisions set by your drawings or local code — this tool counts and weighs, it does not design.

What is the Rebar Calculator?

The ByteTools Rebar Calculator lays out a reinforcing mat for a slab or footing and counts what it takes.

  • Bars each way = (clear span ÷ spacing) + 1, with concrete cover deducted at both edges
  • Lap splices counted automatically when a run is longer than one stock bar
  • ASTM A615 imperial masses (#3 to #8) and metric bar masses from 10 to 25 mm
  • Weight shown in both pounds and kilograms, with a cost estimate
  • Tie intersections and total tie-wire length for the crew
  • Runs entirely in your browser with nothing uploaded

How to use the Rebar Calculator

  1. 1

    Pick imperial or metric and enter the slab length and width.

  2. 2

    Choose the bar size and set the spacing in each direction, plus the concrete cover at the slab edges.

  3. 3

    Set the lap splice length as a multiple of bar diameter and the stock bar length your supplier sells.

  4. 4

    Add a waste allowance, a price per pound or kilogram, and the tie wire you use per intersection.

  5. 5

    Read the bars each way, splice count, total length, weight and the stock bars to buy.

About the Rebar Calculator

The ByteTools Rebar Calculator lays out a reinforcing mat for a slab or footing and counts what it takes. Enter the slab dimensions, the bar size and the spacing each way, and it returns the number of bars in each direction, the lap splices needed when a run exceeds a stock bar, the total linear footage and the weight from the standard bar-size mass table.

Weights come from the ASTM A615 nominal masses — 0.376 lb per foot for a #3 rising to 2.670 for a #8 — and their metric equivalents, where mass per metre is 0.006165 times the diameter squared, giving 0.617 kg/m for a 10 mm bar and 3.853 for a 25 mm. Cover, spacing and lap length are all yours to set.

Everything is calculated in your browser and nothing is uploaded. This tool counts and weighs steel; it does not design it. Bar size, spacing, cover and development length are engineering decisions that belong to your drawings or your local code, so use this to price and order a mat that someone else has specified.

Frequently asked questions

How do I calculate how much rebar I need for a slab?

For each direction, divide the clear span by the bar spacing and add one. A 20 by 20 ft slab with #4 bars at 12 in centres and 3 in cover needs 21 bars each way — 42 bars of about 19.5 ft, roughly 820 linear feet and around 570 lb of steel once a small waste allowance is added.

How much does rebar weigh per foot?

The ASTM A615 nominal masses are 0.376 lb/ft for #3, 0.668 for #4, 1.043 for #5, 1.502 for #6, 2.044 for #7 and 2.670 for #8. In metric, mass per metre is 0.006165 times the bar diameter squared, so a 12 mm bar is 0.888 kg/m and a 20 mm bar is 2.466.

How long should a rebar lap splice be?

Forty times the bar diameter is the common rule of thumb, so a #4 bar laps 20 in and a #5 laps 25. The real requirement is a development length set by your code and depends on concrete strength, bar coating, spacing and cover — 40d is a starting point, not a substitute for the drawings.

What spacing should rebar be in a concrete slab?

Residential slabs commonly use #3 or #4 bars at 12 to 18 in centres each way, but the correct answer comes from the load, the soil and the slab thickness. This calculator takes whatever spacing you specify and tells you the count, weight and cost that follow from it.

How much rebar tie wire do I need?

Allow 12 to 16 inches of wire per intersection so there is enough to wrap and twist. The tool multiplies your figure by the number of crossings in the mat, which for a large slab quickly runs into hundreds of feet — buy it by the roll.

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