Rebar Calculator
The slab's steel grid solved properly: bar counts from real spacing, splices on 20-foot sticks, and tons you can order off a delivery ticket.
Rebar Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Rectangular two-way grids; no trim bars or openings.
- Nominal weights — real bars vary about ±2%.
- One bar size per grid; mixed mats are separate orders.
In short: The spine slab, 20 × 24 ft with #4 at 18-in centres both ways: 14 + 17 bars, 699.333333 ft of steel, 467.154667 lb — 490.5124 lb with 5% site waste. That is 0.245256 tons of #4 at 0.668 lb/ft.
Formula
bars = ⌊span × 12 ÷ spacing⌋ + 1, each way
bar length = run + splices × lap
steel lb = total ft × lb/ft × (1 + waste)
lb/ft is fixed by size: #4 = 0.668, in any grade
[('the grid', 'one bar per space, plus the edge'), ('the lap', 'splices are ordered steel too'), ('the table', 'ASTM A615 nominal weights'), ('grade', 'yield strength — weight-blind')]
Worked Example
- Count bars each way: one per space, plus the edge bar.
- Add splices where runs exceed stock sticks, at your lap.
- Total the run and multiply by the size's lb/ft.
- Apply site waste and state the tonnage for the yard.
- Separate the size decision from the grade decision.
20 × 24 ft, #4 at 18 in: 14 bars running 24 ft (one 20-in lap each on 20-ft sticks) and 17 running 20 ft. Total 699.333 ft × 0.668 = 467.155 lb, 490.512 lb with 5% waste — 0.2453 tons of #4.
Strengths & Limits Of This Model
Where this engine is strong
- Splices and laps in the arithmetic, not the footnote
- Grade/size distinction stated to stop a classic order error
- Tonnage the yard can quote against
Where it stops
- Rectangles only
- No chairs, mesh or stirrups
Practical Use Cases
Slab grids
The residential 12-18 inch decision.
Delivery orders
Tons the steel yard prices.
Spacing trades
What halving the centres really costs.
Driveway mats
Bar counts without a spreadsheet.
Spec checks
Lap and waste made explicit.
Methodology & Editorial Standards
Bar counts use floor(span × 12 ÷ spacing) + 1 each way; bar length adds splices (one per stock-length shortfall) times the entered lap. Weights are the ASTM A615 nominal table — #3 0.376, #4 0.668, #5 1.043, #6 1.502, #8 2.670 lb/ft — applied to total run with the waste allowance. Spacing wider than the span (a one-bar grid) is refused, and the lap is never asserted in prose: it is the spec's number, entered.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Rebar Calculator — 10 Expert FAQs
10 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How much rebar do I need for a slab?
Count bars each way — one per space plus the edge bar — multiply by their lengths including laps, then by the size's weight. The spine slab: 699.3 ft of #4, 490.5 lb with waste.
What does #4 rebar weigh per foot?
0.668 lb/ft — a 20-ft stick is 13.36 lb. #3 is 0.376, #5 is 1.043, #6 is 1.502 and #8 is 2.670: the number is the diameter in eighths of an inch.
What size rebar for a 4-inch slab?
#4 at 12-18 inch centres is the residential band, #3 appearing on light-duty work. Size and spacing come off the drawing; the arithmetic here prices whichever pair you enter.
Does rebar grade change its weight?
No — grade is yield strength (40, 60, 80 ksi), and the weight per foot is fixed by size alone. A #4 is 0.668 lb/ft whether it is Grade 40 or Grade 80.
How much lap do I need at a splice?
Your specification names it — tension laps depend on the bar, the concrete and the exposure, and this page deliberately only prices what you enter. The 16-24 inch chips are common residential territory, not a rule.
Why does spacing matter more than area?
Because the count is one bar per space: 12-in centres need 1.5 times the bars of 18-in on the same slab. Spacing is the money lever; area is just the stage.
How many bars in a 20x24 slab at 18 inches?
14 running the 24-ft direction (240 ÷ 18 = 13 spaces + 1 edge) and 17 the other way — 31 sticks of pattern before laps and offcuts.
Should I order rebar by the bar or by the ton?
The yard sells weight; the drawing counts bars. Compute the run, convert with the size's lb/ft, add waste for offcuts — then order tons and cut to the pattern.
Do I need chairs or supports?
Steel at the bottom of the slab reinforces nothing above a crack — chairs hold the grid at its design height during the pour. They are cheap against the rebar and the pour they protect.
Can mesh replace a rebar grid?
Welded wire is common on thin residential slabs and comes in sheets or rolls; it behaves differently at laps and curls more easily if poorly supported. The grid math here prices rebar; mesh is its own take-off.