Free Calculator · By SubcontractorHub Editorial Team · Published November 2026
Rebar Calculator for Slab and Wall Bar Counts
A rebar calculator counts bars from the span and the spacing you type, then multiplies linear feet by a nominal weight. A 20 by 20 foot slab at 12 inch spacing, both ways, is 21 bars each way, 840 linear feet, and 561.120 pounds of number 4 bar before any extra feet. The pounds per foot come from a fetched table. The spacing does not.
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Book a DemoHow the Rebar Calculator Counts
Pieces use both edges. Weight uses the nominal pounds per foot for the size you leave selected.
Spacing, the extra percent, and the 20 foot stock length are inputs. The weight per foot is the Caltrans nominal figure for the size you pick. The extra percent is not a lap splice.
21 bars running the length, each 20.00 ft. 21 bars running the width, each 20.00 ft. 42 pieces and 840.000 linear feet before extra. #4 at 0.668 lb/ft is 561.120 pounds. Stock sticks at 20.0 ft: 42, rounded up.
All calculations are estimates based on historical information and should be verified by the user.
Important: weight is nominal, spacing is yours
All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.
What This Rebar Calculator Is Not
Poured volume is the concrete calculator. A wall of masonry units is the concrete block calculator. Footing yards, before anyone counts steel, are the concrete footing calculator. This page does not turn a slab thickness into cubic yards, and it does not turn a block into a bar.
The weights are Caltrans Bridge Design Details 13.1, Table 13.1.1, fetched 2026-11-05. The chapter says the details are for ASTM A706 Grade 60 reinforcement with a nominal yield strength of 60.0 kips per square inch. The table is titled ASTM standard reinforcing bar data. A number 3 bar is 0.376 pounds per foot. A number 4 is 0.668. A number 5 is 1.043. A number 6 is 1.502. A number 7 is 2.044. A number 8 is 2.670. Larger sizes through number 18 are in the same dropdown because they are on the same table. The table also prints a nominal diameter. This calculator uses the weight column, not the diameter, for pounds.
| Bar | Nominal diameter (in) | Weight (lb/ft) |
|---|---|---|
| #3 | 0.375 | 0.376 |
| #4 | 0.500 | 0.668 |
| #5 | 0.625 | 1.043 |
| #6 | 0.750 | 1.502 |
| #7 | 0.875 | 2.044 |
| #8 | 1.000 | 2.670 |
| #9 | 1.128 | 3.400 |
| #10 | 1.270 | 4.303 |
| #11 | 1.410 | 5.313 |
| #14 | 1.693 | 7.650 |
| #18 | 2.257 | 13.600 |
Worked slab at number 4
Length 20 feet, width 20 feet, spacing 12 inches, both directions, extra percent zero, stock length 20 feet. Each side is 240 inches. 240 divided by 12 is 20 spaces. Adding the bar at the far edge makes 21 bars. Each of those bars is 20 feet, so one direction is 420 linear feet and both directions are 840.000 linear feet. At 0.668 pounds per foot that is 561.120 pounds. Dividing 840 by a 20 foot stick and rounding up is 42 sticks. Twelve inches and 20 feet are the example inputs. They are not a spacing or a mill length printed on Table 13.1.1.
How should you write the steel order?
- Slab or wall, and the length and width or height you measured.
- The spacing you typed, in inches, and whether both directions were counted.
- The bar size and the pounds per foot from the table, not a shop guess.
- Linear feet before extra, then the extra percent if you typed one. Say it is not a splice.
- Pounds, then stick count only if you typed a stock length. Say the stick count is rounded up.
- A note that cover, chairs, and lap length were not calculated.
Edges Are Included Once
The count is the number of spaces, plus one. A wall or a slab that starts a bar at one edge and ends a bar at the other edge needs that extra bar. If your drawing stops the steel short of the edge, do not use this count. Type a shorter span, or do not add the edge bar and say so on the order. The formula on the screen always adds the edge bar. It will not silently match a drawing that omits it.
Horizontal wall bars are optional. Leave that spacing blank and the page counts verticals only. Each vertical is the wall height you type. It does not add a hook. A hook is extra length the table does not print, so it belongs in the extra-percent box only if you have already turned the hook into a percent yourself.
Two mats are not a checkbox. Run the count and double the linear feet yourself if the drawing shows a top mat and a bottom mat of the same spacing. Doubling inside the tool would hide a drawing that is not two identical mats. The concrete volume under those mats stays on the concrete calculator. This rebar calculator never divides by 27.
Frequently Asked Questions
How does the rebar calculator count bars?
Count equals the span in inches divided by the spacing in inches, with the fraction dropped, plus one bar so both edges are included. A 20 foot side at 12 inch spacing is 240 divided by 12, which is 20 spaces, plus one, which is 21 bars. That count is geometry. It is not a code spacing.
Where do the pounds per foot come from?
Nominal weights are Table 13.1.1 in Caltrans Bridge Design Details 13.1, September 2025, fetched for this page. The table is ASTM standard reinforcing bar data for ASTM A706 Grade 60. A number 4 bar is 0.668 pounds per foot. A number 5 bar is 1.043. The page does not invent a lap length.
Is the extra percent a splice?
No. The extra percent is linear feet you type. The Caltrans table that was fetched prints diameter, area, and weight. It does not print a lap-splice length for a slab. Leave the box at zero if you are not adding extra feet.
Is this the concrete calculator?
No. The concrete calculator is poured volume in cubic yards. The concrete block calculator counts masonry units. This rebar calculator counts bars, linear feet, and weight from the nominal table. Footing volume is the concrete footing calculator.
Put the Bar Size on the Order
Pieces, linear feet, the size, and the pounds per foot you used are the line the yard can check.
Get a Free DemoAll calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.