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Rebar Calculator · https://calcnaut.com/rebar-calculator/

Rebar Calculator

Enter your slab size and rebar spacing to get the total length of rebar, how many standard 20 ft bars to buy, the grid layout, and the approximate weight. Measure in feet and inches, and the formula is shown so you can check the math.

Inputs

Enter your measurements

ft
in
ft
in
in O.C.
On-center spacing of the grid. 12 to 18 inches is typical for residential slabs.
in
Concrete cover from the slab edge. 2 to 3 inches is typical so the steel stays protected.
#
Bar number. #3 to #5 covers most residential slabs and footings.
$
Optional. Enter the price of one 20 ft bar to estimate the rebar cost.

This is an estimate, not professional advice. Check your inputs and verify the result against your plans and local building code before you build or order. See terms and disclaimer.

How this calculator works

Bars one way = floor(usable width(in) ÷ spacing) + 1Usable = dimension(in) − 2 × edge coverTotal length (ft) = (bars one way × usable length) + (bars other way × usable width)Weight = length × lb/ft (#3 = 0.376, #4 = 0.668, #5 = 1.043, #6 = 1.502)

Enter your dimensions and the result updates instantly. A waste allowance is included so you order slightly over rather than running short mid-pour, and ready-mix is rounded up to the nearest quarter yard, which is how it is sold.

Worked example

A 12 ft × 12 ft slab with #4 bar at 12 inches on center and 3 inches of edge cover. Inputs: Slab length 12 ft, Slab width 12 ft, Bar spacing 12 in O.C., Edge cover 3 in, Rebar size 4 #. Result: 14 .

Rebar for common slab sizes

Calculated for #4 bar at 12 inches on center with 3 inches of edge cover. Tap a size to load it above.

Slab sizeTotal rebar length (ft)20 ft bars to buy
8 × 10 ft1518Use →
10 × 10 ft19010Use →
12 × 12 ft27614Use →
16 × 16 ft49625Use →
20 × 20 ft78039Use →
24 × 24 ft1,12857Use →

Method & assumptions

Slab rebar is laid as a grid. We work out how many bars run in each direction by dividing the usable span by your on-center spacing and adding one (a bar at each end), then multiply each count by the length those bars span. The usable span is the slab dimension minus twice the edge cover, because the steel stops short of the edge so the concrete can protect it.

Bar weight comes from the ASTM A615 unit weights: a #3 bar is 0.376 lb per foot, #4 is 0.668, #5 is 1.043 and #6 is 1.502. We multiply the total length by the figure for your bar size. Rebar is usually stocked in 20 foot lengths, so we also divide the total length by 20 and round up to tell you how many sticks to buy. The grid ties figure is simply the number of intersections, which is handy for ordering tie wire.

For most residential slabs, #3 or #4 bar at 12 to 18 inches on center is typical, but always follow your engineer or local code for anything structural. Place the grid on chairs so it sits in the middle third of the slab thickness, where it does the most to control cracking.

Pro tips and common mistakes

  • Keep the steel off the ground. Rebar laying on the subgrade does almost nothing. Set it on chairs or dobies so it sits in the middle third of the slab, where it actually controls cracking.
  • Respect the edge cover. Keep 2 to 3 inches of concrete between the bar and any face. Steel too close to the surface rusts, expands and spalls the concrete off.
  • Lap your splices. Where bars meet end to end, overlap them about 40 bar diameters (roughly 20 inches for #4) and tie them. Butt joints do not transfer load.
  • Tie the intersections. Use the grid ties figure to buy tie wire. Tying every intersection (or every other one) keeps the grid from shifting during the pour.
  • Match the bar to the job. Patios and walkways are fine with #3 to #4 and welded wire mesh. Driveways and structural slabs usually want #4 or #5 on a tighter grid, per code.
How we check this
Every formula is shown in full and verified with automated tests against manufacturer spec sheets and ASTM and ACI standards. We show our work so you can check it.

Frequently asked questions

How much rebar do I need for a 10x10 slab?
A 10 ft × 10 ft slab with #4 bar at 12 inches on center and 3 inches of edge cover needs about 190 linear feet of rebar, which is 10 standard 20 ft bars, in a 10 by 10 grid.
What size rebar for a concrete slab?
Most residential slabs use #3 or #4 bar. Patios and walkways are fine with #3; driveways and slabs carrying load usually use #4 or #5 on a tighter grid. Follow your engineer or local code for structural work.
What spacing should rebar be in a slab?
12 to 18 inches on center is typical. Tighter spacing (12 inches) gives better crack control for driveways and heavier loads; 16 to 18 inches is common for light residential slabs.
How long are standard rebar lengths?
Rebar is most commonly stocked in 20 foot lengths in the U.S. (some yards carry 10, 30, 40 and 60 foot bars). This calculator divides your total length by 20 and rounds up.

References

Related calculators

Concrete Slab Calculator→Concrete Footing Calculator→Concrete Block Calculator→

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