To calculate dowel bar quantity, take the length of each doweled joint (or wall), subtract the edge distance at each end, divide by the spacing, round down and add one. Then multiply the count by the length of each dowel and its mass per metre to get the weight. For a closed loop such as a footing perimeter, the count is simply the perimeter divided by the spacing, rounded up.
The arithmetic is quick. What trips people up is counting the right joints, using the right length for bent dowels and using the right unit weight, because smooth dowels and rebar dowels weigh differently. This post works through both.
Which dowels are you counting?
"Dowel" covers two different products, and the calculation inputs differ:
- Smooth joint dowels are plain round bars across slab and pavement joints. They transfer load across the joint while letting the slabs move. Their size is usually given as a diameter (for example 25 mm or 1 in) and a length, at a spacing along the joint.
- Rebar dowels are deformed bars (10M, 15M, 20M and up) that tie one pour to the next: footing to wall, wall to slab, slab to existing concrete, column to footing. They may be straight or bent (L-bars, hooked bars).
Our post on dowel bars vs rebar explains when each is used. For the count you need four numbers from the drawings: joint or wall length, spacing, edge distance to the first dowel, and the dowel's size and length.
Counting dowels along a joint
For an open run with a defined start and end (a joint across a slab, a doweled edge, one wall):
Number of dowels = ((L − 2e) ÷ s), rounded down, + 1
where L is the joint length, e is the distance from each end to the first dowel and s is the spacing.
Example. A doweled construction joint 4,000 mm long, dowels at 300 mm, first dowel 150 mm from each edge:
- Length available: 4,000 − (2 × 150) = 3,700 mm
- Spaces: 3,700 ÷ 300 = 12.33, round down to 12
- Dowels: 12 + 1 = 13
The last dowel lands 250 mm from the far edge, a little more than the 150 mm edge distance, which is fine. Some details instead spread the dowels evenly so both end distances match; the count is the same.
Why the edge distance matters. On slab joints, dowels usually stop short of where two joints cross, so they do not clash. If you count each joint line as one long run you will overcount. Count per panel edge, the way the joint layout drawing shows it.
Closed loops. For dowels around a continuous perimeter, such as wall dowels on a rectangular footing, there is no start or end:
Number of dowels = perimeter ÷ spacing, rounded up
A 10 m × 12 m footing perimeter (44 m) with dowels at 400 mm gives 44,000 ÷ 400 = 110 dowels. Counting wall by wall with a dowel at every corner gives the same answer once shared corners are counted only once.
Add any extra dowels the details show: at corners, at each side of openings, at pilasters, or doubled at re-entrant corners. These rarely show in a simple spacing callout but they do appear in the typical details.
Dowel length and bent dowels
Smooth dowels have a stated length, centred on the joint. The quantity of steel is just count × length.
Straight rebar dowels get their length from the embedment on each side of the joint. For a dowel set into a footing and lapped with wall bars, that is the embedment into the footing plus the projection above it. Our post on rebar dowel embedment depth explains where those lengths come from.
Bent dowels (L-bars and hooked bars) are weighed along their full developed length, including the bend. An L-dowel with a 300 mm foot and a 900 mm vertical leg is roughly 1,200 mm of bar, less a small allowance for the bend that the detailer works out under RSIC practice. For estimating, adding the leg lengths is close enough; for the bar list, use the detailed length.
Dowel bar unit weight
Weight is count × length (m) × mass per metre (kg/m). The mass per metre depends on whether the dowel is a rebar or a plain round bar.
Rebar dowels use the CSA G30.18 nominal masses, the same values fabricators sell on:
| Bar | Mass per metre | 600 mm dowel | 900 mm dowel | 1,200 mm dowel |
|---|---|---|---|---|
| 10M | 0.785 kg/m | 0.47 kg | 0.71 kg | 0.94 kg |
| 15M | 1.570 kg/m | 0.94 kg | 1.41 kg | 1.88 kg |
| 20M | 2.355 kg/m | 1.41 kg | 2.12 kg | 2.83 kg |
| 25M | 3.925 kg/m | 2.36 kg | 3.53 kg | 4.71 kg |
Smooth round dowels are calculated from the steel density (7,850 kg/m³). A handy form is mass per metre (kg/m) = 0.00617 × d², with d in millimetres. Values below are calculated, rounded:
| Diameter | Mass per metre | Per foot | 450 mm dowel |
|---|---|---|---|
| 20 mm | 2.47 kg/m | about 1.66 lb/ft | 1.11 kg |
| 25 mm | 3.85 kg/m | about 2.59 lb/ft | 1.73 kg |
| 1 in (25.4 mm) | 3.98 kg/m | about 2.67 lb/ft | 1.79 kg |
| 1.25 in (31.8 mm) | 6.22 kg/m | about 4.18 lb/ft | 2.80 kg |
| 32 mm | 6.31 kg/m | about 4.24 lb/ft | 2.84 kg |
| 1.5 in (38.1 mm) | 8.95 kg/m | about 6.01 lb/ft | 4.03 kg |
The 450 mm length is only an example to show the arithmetic; use the length on your drawings. Note that a 25 mm smooth dowel and a 25M rebar are not interchangeable in the weight calculation: 25M has a nominal mass of 3.925 kg/m, set by its nominal area of 500 mm².
Coatings (epoxy or galvanizing) add a little mass that is not in the nominal values.
Worked examples
Example 1: dowels in a shop floor
A 12 m × 24 m slab is cast in 4 m × 4 m panels. The drawings call for smooth 25 mm dowels, 450 mm long, at 300 mm, first dowel 150 mm from each panel edge, at every interior joint.
- Dowels per 4 m panel edge: 13 (from the calculation above).
- Joints parallel to the 24 m side: 2 lines, each crossing 6 panels = 12 panel edges.
- Joints parallel to the 12 m side: 5 lines, each crossing 3 panels = 15 panel edges.
- Total panel edges: 27, so 27 × 13 = 351 dowels.
- Weight: 351 × 0.45 m × 3.85 kg/m = about 608 kg.
Many floors on ground use sawcut joints without dowels and dowel only the construction joints, so count only the joints the drawings mark as doweled.
Example 2: footing to wall dowels
A rectangular footing 10 m × 12 m, with 15M L-dowels at 400 mm into the wall above, each about 1,200 mm developed length.
- Count: 44,000 mm ÷ 400 = 110 dowels.
- Total length: 110 × 1.2 m = 132 m.
- Weight: 132 × 1.570 = about 207 kg.
Add corner and opening extras from the details. For sizing and spacing rules for these dowels, see footing to wall dowels.
Example 3: drilled dowels into existing concrete
Doweling a new slab into an existing foundation wall 8.5 m long with 15M dowels at 600 mm, first dowel 300 mm from each end, each dowel 750 mm long:
- Count: (8,500 − 600) ÷ 600 = 13.2, round down to 13, plus 1 = 14 dowels.
- Weight: 14 × 0.75 × 1.570 = about 16.5 kg.
The steel is the small part of a drilled-dowel job. Hole size, depth and adhesive drive the work; see how to dowel rebar into existing concrete and rebar dowel hole size.
Common counting mistakes
- Counting joint lines instead of panel edges, which ignores the gaps at joint intersections.
- Forgetting the "+ 1" on open runs, or adding it on closed loops where it double-counts a corner.
- Using straight length for bent dowels. The hook or foot is real steel and is on the invoice.
- Mixing up smooth and rebar unit weights, especially between 25 mm round bar and 25M.
- Missing extras at corners, openings, steps and pilasters shown only in the typical details.
- Assuming every joint is doweled. Check the joint plan.
A dowel count from a calculator is fine for a budget. For ordering, the count should come from a take-off of the actual drawings, including laps, extras and bent shapes.
Getting dowels on the order
We fabricate rebar dowels, straight and bent, cut to length and tagged by mark with the rest of the bar list, and deliver across the GTA and Ontario. Send the foundation and slab drawings and we will do the count as part of a free take-off, so the dowels, laps and corner bars are all on one order.
Request a take-off and quote, or see our rebar products for straight bar, fabricated dowels and stirrups, and GFRP.
Frequently asked questions
How do you calculate the number of dowel bars in a joint?
Subtract the edge distance at each end from the joint length, divide by the dowel spacing, round down and add one. A 4,000 mm joint with dowels at 300 mm and the first dowel 150 mm from each edge gives (4,000 minus 300) ÷ 300 = 12.3, so 12 spaces and 13 dowels.
How do you calculate dowel bar weight?
Multiply the number of dowels by the length of each one in metres and by the mass per metre. For rebar dowels use the CSA G30.18 nominal mass (1.570 kg/m for 15M). For smooth round dowels, mass per metre is about 0.00617 times the diameter in millimetres squared.
What is the unit weight of a 25 mm dowel bar?
A 25 mm round steel bar weighs about 3.85 kg per metre, based on a steel density of 7,850 kg per cubic metre. A 25M rebar is a different product with a nominal mass of 3.925 kg/m.
How long should a dowel bar be?
The drawings set it. Smooth joint dowels have a specified length centred on the joint. Rebar dowels need enough embedment on each side to develop the bar or meet the code minimum, so their length comes from the embedment and any hook or bend.
