Rebar Calculator
Grid layout, linear feet, lap splices and weight — for slabs, pads and footings.
Rebar to order
573 ft
- Bars lengthwise
- 14 × 19.5 ft
- Bars widthwise
- 14 × 19.5 ft
- Net length
- 546 ft
- Lap splices
- None — runs fit a single stock bar
- Stock bars (20 ft)
- 29 bars
- Weight
- 383 lb · 0.19 tons
- Steel per sq ft
- 1.37 ft/sq ft
- Tie intersections
- 196 · 147 ft of wire
- Chairs
- 25 chairs
Bar size, spacing, cover and lap length are structural decisions governed by code and by the engineer of record — this sizes and prices a specified mat, it does not specify one.
Pricing the whole pour? Use the concrete slab cost calculator →
How this is calculated
run length = slab dimension − 2 × cover bars each way = floor(opposite run / spacing) + 1 lap length = lap multiple × bar diameter pieces/run = ceil((run − lap) / (stock − lap)) material = run + (pieces − 1) × lap order = material × (1 + waste%) weight = order × lb per foot
Bar counts add one because both edges of the mat get a bar, which is the most common off-by-one in hand estimates. Splices are only added where a run is longer than a stock bar, and the overlap is counted as extra material rather than extra bars — you buy the length, not the pieces. Weights are the standard US figures: 0.376, 0.668, 1.043 and 1.502 lb per foot for #3 through #6.
Bar sizes and weight
| Size | Diameter | Weight | 40× lap | Typical use |
|---|---|---|---|---|
| #3 — 3/8" | 0.375" | 0.376 lb/ft | 15" | Light slabs, tie beams |
| #4 — 1/2" | 0.500" | 0.668 lb/ft | 20" | Residential slabs, footings |
| #5 — 5/8" | 0.625" | 1.043 lb/ft | 25" | Driveways, thicker slabs |
| #6 — 3/4" | 0.750" | 1.502 lb/ft | 30" | Heavy slabs, walls |
| #7 — 7/8" | 0.875" | 2.044 lb/ft | 35" | Structural, commercial |
| #8 — 1" | 1.000" | 2.670 lb/ft | 40" | Columns, heavy structural |
Steel per 100 sq ft by spacing
| Spacing | Linear ft per 100 sq ft | #4 weight | #5 weight |
|---|---|---|---|
| 12 in | 200 ft | 134 lb | 209 lb |
| 16 in | 150 ft | 100 lb | 156 lb |
| 18 in | 133 ft | 89 lb | 139 lb |
| 24 in | 100 ft | 67 lb | 104 lb |
These are steady-state figures for a large mat and run slightly under what the calculator reports for a small slab, where the extra bar at each edge is a bigger share of the total. Note that going from 18 to 12 inch spacing adds half again as much steel.
Frequently asked questions
How much rebar do I need for a slab?
For a typical residential slab, #4 bar on 18 inch centres each way is the common specification, which works out around 1.4 linear feet of bar per square foot of slab. A 20 by 20 pad therefore takes roughly 550 feet, or 28 twenty-foot bars before waste. Tighter spacing or heavier bar is a structural decision, not a preference.
What spacing should rebar be?
Twelve, sixteen or eighteen inches on centre covers most residential work, with 18 inches the usual default for a four inch slab. Halving the spacing roughly doubles the steel, so it is the single biggest cost lever in the mat — which is exactly why it should come from an engineer or the local code rather than from a calculator.
How long should a rebar lap splice be?
A common rule of thumb is 40 times the bar diameter for Grade 60 in ordinary conditions, so 20 inches for #4 and 25 for #5. Real lap lengths depend on concrete strength, bar coating, spacing and whether the bar is in tension — the rule of thumb is for estimating material, not for detailing what actually gets tied.
How much does rebar weigh?
Per linear foot: #3 is 0.376 lb, #4 is 0.668, #5 is 1.043 and #6 is 1.502. The quick mental version is that the bar number squared, divided by 600, is close to the weight in pounds per foot. Suppliers often price by the ton, so weight rather than length is what you need to compare quotes.
Do I need rebar chairs?
Yes, for any slab where the steel is meant to do something. Rebar lying on the subgrade is below the concrete's neutral axis and contributes almost nothing; it needs to sit in the middle to upper third. Pulling the mat up as you pour is the traditional method and it is not reliable — chairs at three to four foot intervals cost very little and hold the position.
Should I use rebar or wire mesh?
Mesh is adequate for a plain four inch residential slab and costs far less to buy and place. Rebar earns its keep in thicker slabs, on poor or expansive soil, under vehicle loads, and anywhere the slab has to span a soft spot. Neither stops cracks forming — they hold the crack closed so the slab keeps working.
Results are planning and take-off estimates, not structural engineering. Bar size, grade, spacing, cover, lap length and placement are set by code, by soil conditions and by the engineer or designer responsible for the slab. Lap lengths in particular depend on concrete strength and bar condition — use the specified detail, not the rule of thumb here.