REBAR ORDER

Enter your slab length and width in feet, the bar spacing on center (12" for driveways and heavy loads, 18" for patios and garage floors, 24" for sidewalks), and the bar size (#3 light-duty, #4 residential standard, #5 driveways and heavy loads).

The calculator returns bar count in each direction, total linear feet, total weight, and the number of 20-foot sticks to order from the supply yard. Standard residential rebar comes in 20-foot lengths for delivery; tie wire and chairs to support the rebar above the subgrade are sold separately.

For a 20 × 12 ft patio at 18" o.c. spacing with #4 rebar:

Bar spacing in feet: 1.5 ft. Bars in long direction: ⌈12 ÷ 1.5⌉ + 1 = 9 bars × 20 ft = 180 lin ft. Bars in short direction: ⌈20 ÷ 1.5⌉ + 1 = 15 bars × 12 ft = 180 lin ft. Total: 360 lin ft.

20-foot sticks: ⌈360 ÷ 20⌉ = 18 sticks. Total weight at 0.668 lb/ft: 240 lbs of #4 rebar.

At ~$8–$15 per 20-ft stick of #4, materials cost $150–$270. Add $30–$50 in tie wire and rebar chairs (the little plastic supports that hold the grid 1.5–2 inches above the gravel base for proper concrete cover).

For a 30 × 20 ft driveway at 12" o.c. with #5 rebar (vehicle loads): 51 bars long × 30 ft + 31 bars short × 20 ft = 2,150 lin ft = 108 sticks. At $12–$20 per #5 stick: $1,300–$2,160 in rebar alone.

Skipping rebar chairs. Rebar laid directly on the gravel base ends up at the bottom of the slab, where it does nothing. Use plastic chairs or dobie blocks to keep the grid 1.5–2 inches above the bottom.

Wrong bar spacing for the load. 24" o.c. is for sidewalks and shed pads. Driveways need 12–16" o.c. with #4 or #5. Mismatched spacing cracks under load.

Missing the perimeter dowels. Slabs that meet existing concrete (foundations, sidewalks, garage floors) need #4 dowels drilled into the existing concrete every 12–18 inches to tie the new pour to the old.

Forgetting concrete cover requirements. ACI 318 requires 3 inches of cover from the earth side and 1.5 inches from the finished surface. Rebar too close to the surface rusts and spalls the concrete within years.

Spacing by use: sidewalks 24" o.c., patios 18" o.c., garage floors 16" o.c., driveways 12" o.c.

Bar size by use: #3 light-duty (1/8 lb/ft), #4 residential standard (0.67 lb/ft), #5 driveways and heavy slabs (1.04 lb/ft).

Standard length: 20 ft sticks. Lap splices needed at any joint: 40× bar diameter (16 inches for #4, 20 inches for #5).

Cover requirements (ACI 318): 3 inches from earth side, 1.5 inches from finished surface.

Tie wire: about 1 lb per 100 linear feet of rebar to tie the grid intersections.