How this calculator works
Bars run in both directions. We work out how many bars fit across each dimension at your spacing (leaving the edge cover clear), multiply by each bar's length, add a waste allowance for laps and cuts, then divide by your stock length.
Worked example: a 20 × 12 ft slab at 16 in spacing with 3 in edge cover: 9 bars run the long way (each 19.5 ft) and 15 run the short way (each 11.5 ft). That is 348 linear ft, or about 383 ft with 10% waste, which is 20 twenty-foot bars of #3, weighing 144 lb.
Assumptions and sources
- Weight per foot: #3 = 0.376 lb, #4 = 0.668 lb, #5 = 1.043 lb. Support the grid on chairs so it sits in the middle of the slab, not on the ground.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
Tips before you buy
- Rebar placement matters as much as quantity. Keep it supported mid-depth with chairs, since steel resting on the dirt does little.
- Splice (lap) bars by about 40 bar diameters, so roughly 15 in for #3 and 20 in for #4, and tie each lap with wire.
- Many residential slabs use #3 or #4 bars at 16 to 24 in centers, or welded wire mesh. Garage floors, driveways and structural slabs may need engineered designs.
Tools & materials for this project
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Frequently asked questions
What spacing should rebar be in a slab?
Typical residential slabs use 16 to 24 inch centers each way, depending on thickness, load and local practice. Heavier loads call for closer spacing. Check your plans or an engineer.
Is rebar or wire mesh better?
Rebar is stronger and better for heavy loads and footings. Welded wire mesh is quick to install for light slabs. Neither prevents cracking, but both keep cracks tight.
How much does #3 rebar weigh?
0.376 lb per foot, so a 10 ft stick weighs about 3.8 lb. #4 is 0.668 lb per foot and #5 is 1.043 lb per foot.
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Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.