The Slab and Footing tabs add the laps into the total length automatically.
How the rebar calculator works
RebarMath lays out a grid. In each direction the number of bars is the clear span (the slab dimension minus edge cover on both sides) divided by the spacing, rounded down, plus one; each bar is as long as the other dimension minus cover. A run longer than a stock bar is made of lapped pieces: each extra piece adds one lap of 40 bar diameters (20 in for #4) by default. The total length plus waste, divided by the stock length, gives the number of bars to order, and weight uses the standard ASTM A615 unit weights. Footings use continuous bars along the length plus optional cross bars.
Rebar sizes, weights and laps
| Bar | Diameter | Weight per ft | 20 ft bar | Lap at 40 db |
|---|---|---|---|---|
| #3 | 0.375 in | 0.376 lb | 7.52 lb | 15 in |
| #4 | 0.5 in | 0.668 lb | 13.36 lb | 20 in |
| #5 | 0.625 in | 1.043 lb | 20.86 lb | 25 in |
| #6 | 0.75 in | 1.502 lb | 30.04 lb | 30 in |
| #7 | 0.875 in | 2.044 lb | 40.88 lb | 35 in |
| #8 | 1 in | 2.67 lb | 53.4 lb | 40 in |
The 40-diameter lap is a common rule of thumb for slabs on grade; structural laps under ACI 318 depend on concrete strength, bar position, coating and cover and can be much longer, and plans or local codes govern. Bar counts here do not optimize cutting, so check your cut list before ordering. Results are estimates for planning, not engineering.
Frequently asked questions
What is the lap length for rebar?
Often 40 to 60 bar diameters; 40 d is a common minimum for slabs on grade. Follow the plans or code where they apply.
How long is a #5 lap splice?
25 in at 40 diameters, 30 in at 48 diameters.
Can I stagger lap splices?
Yes, staggering laps between neighbouring bars is good practice so they do not all fall at one line.
Is RebarMath free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.