You are standing in the building supply aisle and the labels say #3, #4, #6 and #8. Those numbers are not millimeters and they are not a strength rating. Here is what they really mean, how to turn any rebar size into a diameter in seconds, and how to work out how much steel your project will actually weigh.
What the number on rebar actually means
Rebar (short for reinforcing bar) is the steel rod buried inside concrete to help it resist cracking and bending. In the United States, each bar size is named with a number, and that number is simply the diameter of the bar counted in eighths of an inch.
So a #8 bar is 8/8 of an inch, which is exactly 1 inch (25.4 mm) across. A #4 bar is 4/8, or half an inch. To decode any US rebar size, divide the number by 8 and you get the diameter in inches. Multiply by 25.4 if you want millimeters.
Here are the most common sizes you will see on a job site:
- #3: 0.375 in (9.53 mm), cross section 0.11 sq in
- #4: 0.500 in (12.7 mm), cross section 0.20 sq in
- #6: 0.750 in (19.1 mm), cross section 0.44 sq in
- #8: 1.000 in (25.4 mm), cross section 0.79 sq in
Notice something in that list: a #8 bar is only twice as wide as a #4, but its cross section is almost four times bigger (0.79 versus 0.20 sq in). That is because area grows with the square of the diameter. A small jump in bar number means a much bigger jump in the amount of steel.
Canada uses a different system
If you are working from Canadian plans, you will see sizes like 10M, 15M, 20M or 25M instead. In this metric system, the number is the nominal diameter in millimeters, not in eighths of an inch. A 10M bar is therefore nowhere near a US #10, so never swap one label for the other without converting.
How to calculate the weight of rebar for your project
Rebar is often priced and delivered by weight, so knowing the total tonnage helps you order and budget. The method is a single multiplication:
Total weight = total length of bar × weight per unit length of the size you chose
You need two reference values to start:
- #6 bar: 1.502 lb per foot (2.235 kg per meter)
- #8 bar: 2.670 lb per foot (3.973 kg per meter)
Worked example in metric
Say your plan calls for 50 meters of #8 bar. The math is 50 × 3.973 = 198.65 kg of steel. That is close to 200 kg, which is good to know before you try to load it into a pickup truck.
Worked example in imperial
Now imagine a footing that needs 4 runs of #6 bar, each 30 feet long. First add up the length: 4 × 30 = 120 feet. Then multiply: 120 × 1.502 = 180.24 lb, or roughly 82 kg.
Switching that same job from #6 to #8 would push the weight to 120 × 2.670 = 320.4 lb, about 1.8 times heavier for the same length. Bar size has a much bigger effect on your order than most people expect.
Don’t forget the anchorage length
Rebar only works if it is properly anchored. Bars are either embedded deep into the neighboring concrete element or bent and hooked at their ends. The usual guideline is an anchorage length of 40 to 60 times the bar diameter.
For a #8 bar (25.4 mm), that means 40 × 25.4 = 1,016 mm up to 60 × 25.4 = 1,524 mm, so roughly 40 to 60 inches. For a #4 bar (12.7 mm), it works out to 508 to 762 mm, or about 20 to 30 inches. Those extra lengths need to be included in your total before you calculate the weight.
Common mistakes to avoid
- Reading the bar number as millimeters. A #8 bar is 25.4 mm, not 8 mm. The number counts eighths of an inch.
- Confusing size with grade. The grade is a separate marking about strength. Grade 60, the most common in modern American construction, means a minimum yield strength of 60 ksi (420 MPa). Higher grades exist, up to 150.
- Using one fixed anchorage length for every bar. It scales with the diameter, so a thicker bar always needs a longer anchorage.
- Mixing US and Canadian labels. A 10M bar is about 10 mm across, while a #10 bar, by the eighths rule, is 1.25 inches (about 32 mm).
Rebar always goes hand in hand with the concrete around it. Once your steel is sorted out, you can estimate how much concrete the slab or footing will need with the concrete calculator.
