Rebar Calculator
Estimate bar count, spacing, total length, and steel weight for one- or two-layer grids. Choose a target first.
Rebar Calculator — From Spacing to Weight and Cost
Use the calculator above to size, count, and weigh reinforcing bars for slabs, footings, and walls. This guide explains the assumptions behind the math, common bar sizes (#3–#11), lap splices, and quick checks so you can order confidently.
Start here Quick Start: Using the Rebar Calculator
- Select element type (slab, footing, wall) and units (US/metric). Enter plan dimensions and thickness/height.
- Choose your bar size (e.g., #4) and spacing (e.g., 12 in o.c.) for each direction/face. If you’re targeting a steel ratio, use the design approach option.
- Set edge cover and lap splice assumptions. Typical tension lap is 40d–60d; your code/spec governs.
- Review outputs: bar count each way/face, total linear feet (or meters), weight, and an order list with standard stock lengths.
- Use the waste/contingency slider (often 5–10%) before exporting your takeoff.
Choosing Your Method: Spacing-Based vs. Design-Based
Spacing-Based Takeoff
Pick bar size and on-center spacing; the tool computes counts, lengths, and weight from geometry.
- Pros: Fast; matches typical plan notes (e.g., “#4 @ 12″ o.c. each way”).
- Cons: Doesn’t verify required steel area \(A_s\).
Design-Based (Target Steel Ratio)
Enter design loads/strength and target steel ratio; the tool suggests bar size/spacing meeting \(A_s\).
- Pros: Engineering-aligned; checks steel area and provides options.
- Cons: Needs more inputs and assumptions (\\( f’_c, f_y, \phi \\), etc.).
Rule of thumb: Light slabs on grade often use #4 @ 12″ o.c. each way; footings commonly use #4–#5 with 6″–12″ spacing. Walls frequently have two faces of steel with verticals and horizontals.
What Moves the Number: Key Drivers of Rebar Quantity & Weight
Variables & Symbols
| Symbol | Meaning | Typical Units |
|---|---|---|
| \(L, W, H\) | Plan length, width, wall height | ft, m |
| \(t\) | Slab/footing thickness | in, mm |
| \(c\) | Clear cover | in, mm |
| \(s\) | Bar spacing (o.c.) | in, mm |
| \(d_b\) | Bar diameter | in, mm |
| \(L_l\) | Lap splice length | in, mm |
| \(w_f\) | Weight per foot (US) | lb/ft |
| \(w_m\) | Weight per meter (metric) | kg/m |
US bar weights per foot are standardized (approx.): #3 0.376, #4 0.668, #5 1.043, #6 1.502, #7 2.044, #8 2.670, #9 3.400, #10 4.303, #11 5.313 lb/ft. Metric ≈ US×1.488 to get kg/m.
Bar count each direction (slab/footing):
\[ n = \left\lceil \frac{(L – 2c)}{s} \right\rceil + 1 \]
Total length (one direction): \(\,L_{\text{tot}} = n \times (W – 2c) + n_s \times L_l\) (add splices as needed). Weight: \(W = L_{\text{tot}} \times w_f\) (US) or \(L_{\text{tot}} \times w_m\) (metric).
Worked Examples
Example 1 — Slab on Grade (US units)
- Slab: 20 ft × 20 ft × 5 in, cover \(c = 2\) in
- Bars: #4 @ 12 in o.c. each way, single mat
- Lap splice \(L_l = 40d\) (for #4, \(d_b=0.5\) in ⇒ \(L_l = 20\) in), assume 1 splice per bar
\(n_L = \left\lceil \dfrac{L-2c}{s} \right\rceil + 1 = \left\lceil \dfrac{240-4}{12} \right\rceil + 1 = 20\)
Same for \(n_W = 20\). Total bars ≈ 40.
Each L-direction bar spans \(W-2c = 240-4 = 236\) in = 19.67 ft.
Add one splice per bar: \(20\) in = 1.67 ft.
Per bar length ≈ \(19.67 + 1.67 = 21.34\) ft.
Total L-direction footage ≈ \(20 \times 21.34 = 426.8\) ft. Same for W-direction ⇒ \(853.6\) ft.
#4 weight \(w_f = 0.668\) lb/ft ⇒ \(W \approx 853.6 \times 0.668 = 570\) lb.
Example 2 — Wall Two Faces (Metric)
- Wall: length 8.0 m, clear height 3.0 m, thickness 200 mm, cover \(c = 40\) mm
- Bars: 12 mm @ 200 mm o.c. both ways, two faces
- Lap splice \(L_l = 45d = 540\) mm, assume one lap per vertical bar
Verticals per face: \( n_v = \left\lceil \dfrac{8{,}000-2c}{200} \right\rceil + 1 = 40 \).
Horizontals per face over clear height: \( n_h = \left\lceil \dfrac{3{,}000-2c}{200} \right\rceil + 1 = 16 \).
Two faces ⇒ vertical bars 80; horizontal bars 32 per panel length (spanning 8.0 m).
Vertical clear height per bar: \(3.0 – 2c = 2.92\) m + splice 0.54 m ⇒ 3.46 m each.
Total vertical length: \(80 \times 3.46 = 276.8\) m.
Horizontal span per bar: \(8.0 – 2c = 7.92\) m; two faces, 16 bars/face ⇒ \(32 \times 7.92 = 253.4\) m.
Total ≈ \(530.2\) m of 12 mm bar.
12 mm ≈ 0.888 kg/m ⇒ \(W \approx 530.2 \times 0.888 = 471\) kg.
Rounding: Contractors often round up bar counts and add 5–10% to minimize short shipments and accommodate field fitting.
Bar Sizes, Patterns & Variations
Standard US bar sizes and weights, common spacings, and where they’re used.
| Bar | Dia. (in) | Weight (lb/ft) | Typical Spacing | Common Uses |
|---|---|---|---|---|
| #3 | 0.375 | 0.376 | 12–18 in | Light slabs, sidewalks, small footings |
| #4 | 0.500 | 0.668 | 8–12 in | Slabs on grade, grade beams, strip footings |
| #5 | 0.625 | 1.043 | 6–12 in | Heavier slabs, walls, mat footing top layers |
| #6 | 0.750 | 1.502 | 6–12 in | Walls, beams, columns, mats |
| #7 | 0.875 | 2.044 | 6–9 in | Heavier walls, transfer elements |
| #8 | 1.000 | 2.670 | 6–12 in | Mats, beams, columns |
Edge Cases & Variations
- Two-way mats: Top and bottom layers each way (4 directions total). Double your faces/directions accordingly.
- Hooked ends: Replace lap splices with hooks where allowed; adjust length by hook geometry.
- Irregular shapes: Break into rectangles and add perimeter beams; the calculator sums sub-areas.
Buying, Logistics & Practicalities
Choosing Bar Size & Grade
- #4/12 mm is the most common “workhorse” for slabs and footings.
- Grade 60 (420 MPa) is typical; seismic zones may require special detailing or higher grades.
- Match project specs for epoxy coating, stainless, or galvanized in aggressive environments.
Field Logistics
- Order stock lengths (20/40/60 ft or 6/12 m) that minimize waste for your cut list.
- Stage bars by direction/face; label bundles to match plan grids.
- Check cover blocks/chairs and bar supports early—quality here saves rework.
Sanity Checks
- Do: Compare steel ratio to typical ranges for your element and loading.
- Don’t: Ignore development length, hooks, or bar congestion at laps/corners.
Code note: Use project drawings and governing standards (e.g., ACI 318/301) for minimum covers, maximum spacings, and splice rules. This article focuses on takeoff math, not compliance.
FAQs
How do I estimate lap splice length?
Many projects use tension lap splices of \(40d\)–\(60d\) (bar diameter multiples), but actual values depend on bar size, concrete strength, coating, and location (top bars, seismic, etc.). Always follow the project specifications and code.
What’s the weight per foot for common bars?
Approximate US weights (lb/ft): #3 0.376, #4 0.668, #5 1.043, #6 1.502, #7 2.044, #8 2.670, #9 3.400, #10 4.303, #11 5.313. Multiply by 1.488 for kg/m.
How does edge placement affect bar count?
Placing the first and last bar at half-spacing from edges typically results in \( \lceil L/s \rceil + 1 \) bars. If you start at a full spacing from the edge, count decreases by one but coverage suffers.
How much waste should I include?
For simple grids, 5–8% is common; complex geometry, heavy splicing, or short stock lengths may justify 10%+. Review your cut list against available stock to refine.
Can I mix bar sizes in one layer?
Yes, but transitions must be detailed properly. Mixing sizes complicates splices and can cause congestion. The calculator supports mixed patterns if you break the element into zones.
When should I use epoxy-coated or stainless bars?
Use corrosion-resistant bars in deicing salt, marine, wastewater, or other aggressive exposures if specified. Epoxy is common; stainless offers higher durability at a premium.