Rebar Calculator for Slabs (Bars, Length & Weight)

How to use the rebar calculator

1. Enter the slab length and width.
2. Choose the bar size — US sizes #3 to #8 or metric bars — and the spacing each way.
3. Set the edge cover (3 in / 75 mm is usual for slabs on ground).
4. Enter the length of the stock bars you can buy (20 ft in the US; 6 or 12 m in metric countries) and the lap length for joints.

Rebar grid formulas

Bars one way = (Slab width − 2 × cover) ÷ spacing + 1 → round down, then + 1 Bar length = Slab length − 2 × cover Total length = Σ (number of bars × bar length) + laps Weight = Total length × weight per foot (or per metre)

Worked example

A 20 × 10 ft driveway pad with #4 bars at 12 in each way and 3 in cover:

• Lengthwise bars: (120 − 6) ÷ 12 = 9.5 → 9 + 1 = 10 bars of 19.5 ft
• Widthwise bars: (240 − 6) ÷ 12 = 19.5 → 19 + 1 = 20 bars of 9.5 ft
• Total = 195 + 190 = 385 ft × 0.668 lb/ft = 257 lb
• Stock bars: each 19.5 ft piece uses one 20 ft bar (10), and two 9.5 ft pieces come from one 20 ft bar (10) → 20 bars

US rebar sizes and weights

Metric bars use weight (kg/m) = diameter² ÷ 162.2 (diameter in mm), the same formula as our TMT Steel Weight Calculator.

Tips for placing rebar

• Support the mat on chairs or concrete dobies so it sits in the middle of the slab, not on the ground.
• Tie intersections with wire — every other intersection is usually enough for slabs on ground.
• Stagger lap splices so they don’t all fall on the same line.
• Keep bars clean — light surface rust is fine, but remove mud, oil and loose scale.
• Add bars around openings and at re-entrant corners to control cracking.
• Check local requirements: many building departments require an inspection of the rebar before the concrete is poured.