How many a joint takes, what each one weighs, and what the number in the search actually means
← Back to the calculatorEach 3/4 in × 18 in dowel weighs 2.255 lb, so 3 = 6.77 lb, 4 = 9.02 lb, 5 = 11.28 lb, with one sleeve each. If you meant a #4 bar rather than four pieces, that is a 1/2 in dowel — the size table is on the concrete dowels page.
That is the whole arithmetic, and it is why the search reads the way it does. People type “4 rebar dowels” when they have a 4 ft joint, because at 12 in spacing the count and the joint length in feet are the same number.
“4 rebar dowels” is read two ways, and both are common enough that a page has to say which one it is answering.
The count is what you order; the size is what the drawing specifies. Keeping them apart matters, because four #4 dowels and four 3/4 in dowels are different orders, different weights and different joints.
If the question is really the size, the diameter rule and the full table are on the concrete dowels page: diameter is slab thickness ÷ 8, so a 4 in slab takes 1/2 in, a 6 in slab 3/4 in, an 8 in slab 1 in.
The count is the same formula used for any dowelled joint: clear length divided by spacing, rounded down, plus one. The clear length is the joint less the two 6 in end offsets.
So at 12 in on centre the count is the joint length in feet, exactly, for any whole-foot joint. That is the reason 3, 4 and 5 are the numbers people search: they are 3 ft, 4 ft and 5 ft of joint.
The rule holds because both the spacing and the end offset are 12 in. Change the spacing and the identity breaks — which is the table below.
Twelve inches on centre is the slab-on-grade standard. Loosening to 18 in or 24 in cuts the count and the bearing capacity together, so the table is a trade, not a discount.
| Joint length | 12 in o.c. | 18 in o.c. | 24 in o.c. |
|---|---|---|---|
| 3 ft | 3 | 2 | 2 |
| 4 ft | 4 | 3 | 2 |
| 5 ft | 5 | 3 | 3 |
Swipe the table sideways for more columns →
The panel width is the joint length for a joint running across a walkway or an apron. A 4 ft wide walkway takes 4 dowels per joint at 12 in; a 3 ft step takes 3; a 5 ft apron takes 5. Same rule, read sideways.
What you give up at 18 in is bearing: the load per dowel goes up because there are fewer of them, and the joint face sees more pressure per bar. That is why 12 in is the default on slabs that carry vehicles and 18 in shows up on lightly loaded flatwork.
Weight is per dowel, so the total is a multiplication. A 3/4 in × 18 in dowel is 2.255 lb; the smaller and larger sizes move with the square of the diameter.
| Count | 3/4 in × 18 in | 5/8 in × 18 in | 1/2 in × 18 in |
|---|---|---|---|
| 3 dowels | 6.77 lb | 4.71 lb | 3.00 lb |
| 4 dowels | 9.02 lb | 6.28 lb | 4.00 lb |
| 5 dowels | 11.28 lb | 7.85 lb | 5.00 lb |
Swipe the table sideways for more columns →
Ordering is where a small count gets awkward. A 20 ft stick yields 13 dowels at 18 in, so 3, 4 or 5 dowels come out of one stick and leave 8 to 10 dowels' worth of offcut. If those joints are the whole job, buy per piece; if you have other joints, the offcut is already spoken for.
One sleeve per dowel, so 3 dowels take 3 sleeves, 4 take 4, 5 take 5. Only one half of each bar is debonded, and it is the same half on every dowel in the joint — the side that has to slide as the joint opens.
A sleeve on the wrong half is worse than no sleeve at all, because the joint then binds on one side and opens at an angle. Check the direction before the pour; there is no fixing it after.
Past five dowels the one-per-foot rule still works, it just stops being a number you can hold in your head. The full count table, the 60 × 60 ft worked example and the sleeve arithmetic are on the concrete dowels page.
If your joint is longer than 5 ft, the number you want is probably on that page rather than this one.
Small counts are sold per piece at most suppliers, and that is usually the right way to buy them. The alternative — one 20 ft stick cut into 18 in lengths — only makes sense if the rest of the stick has a home on the same job.
Two things to check on a per-piece order: that the length is 18 in and not a nominal 12 in (a 12 in dowel gives 6 in of embedment each side, a third less bearing), and that the sleeve is included or ordered separately. Some listings sell the bar and the sleeve as one item, some do not.
Four, at 12 in on centre. The count is floor((joint length - 12 in) / 12 in) + 1, which for a 4 ft joint is floor(36 / 12) + 1 = 4. The same rule gives 3 for a 3 ft joint and 5 for a 5 ft joint, so at 12 in spacing the count equals the joint length in feet.
Both readings are common. Four pieces is a count - four dowels on a 4 ft joint. A #4 bar is a size - 1/2 in diameter, 1.00 lb per 18 in, and it is the dowel for a 4 in slab. This page covers the count; the size table is on the concrete dowels page.
A 3/4 in x 18 in dowel weighs 2.255 lb. The smaller sizes fall quickly: 5/8 in x 18 in is 1.57 lb, 1/2 in x 18 in is 1.00 lb, 1 in x 18 in is 4.01 lb. Weight is pi/4 x d squared x 18 in x 0.2836 lb/in3.
Thirteen at 18 in, with 6 in left over: 240 in divided by 18 in is 13.33. The count is the same at every diameter because length sets it, not weight. So 3, 4 or 5 dowels all come out of one stick.
12 in on centre is the standard for slabs on grade, with the first dowel 6 in from the free edge. At 18 in on centre the same 3, 4 and 5 ft joints take 2, 3 and 3 dowels; at 24 in they take 2, 2 and 3. Looser spacing means fewer dowels and less bearing capacity per foot of joint.
Yes, one sleeve per dowel. Only one half of each dowel is debonded, so three dowels take three sleeves, not one and a half. The sleeved half is the same side for every dowel in the joint.
Yes. Dowels are sold per piece at most suppliers, and 4 is a normal small order. If you already have 20 ft bar, four 18 in dowels come out of one stick with 168 in left over, so per-piece is usually the cheaper way to buy a count this small.
Four, because the joint is 4 ft long and the dowels run at 12 in on centre. A 3 ft step takes 3 and a 5 ft apron takes 5, which is the same one-per-foot rule.
Because the end dowels sit 6 in in from the free edges, so a 48 in joint only spans 36 in of centres: the dowels land at 6, 18, 30 and 42 in. Five would put one at 0 and one at 48 in, right at the joint faces where there is no concrete on one side to bear against. The same 6 in offset is why a 3 ft joint takes 3 and a 5 ft joint takes 5 — and it is why the count is the joint length in feet rather than the length in feet plus one.