Why the count is one more than you expect
Joists at 16 inches on center across a 20 foot wall come out at 16 pieces, not 15. The wall is 240 inches, 240 divided by 16 is 15, and then there is a joist at the far end that closes the last bay. Every layout that starts with a member at zero and marches to the end has one more member than it has spaces. Miss that and you are one joist short on every floor you frame, which is the single most common arithmetic error in a framing takeoff.
The 19.2 inch option looks strange until you divide 96 by 5. Five spaces of 19.2 inches land exactly on an 8 foot sheet, so subfloor edges fall on framing without a single measurement. It is a real spacing that shows up on tape measures as the black diamonds, and it saves a joist per bay compared to 16 inch centers on floors where the span table allows it.
The parts that are not joists
A floor takeoff that stops at the joist count is short by a quarter. The rim joist closes both ends of the run and is the same stock as the joists. The band joists run down the two outside edges parallel to the joists and are often forgotten because they look like ordinary joists on the drawing. Blocking between the joists is a pile of short pieces cut to the spacing minus the joist thickness, so a 16 inch on center floor takes 14-1/2 inch blocks, and there is one per bay per row.
| Part | How many | Length |
|---|---|---|
| Joists on layout | floor(wall / spacing) + 1 | The span |
| Doubled joist | One per parallel partition above | The span |
| Rim | 2 | The bearing wall length |
| Band | 2 | The span |
| Blocking | (joists − 1) per row | Spacing − 1-1/2 in |
| Hangers | 1 or 2 per joist | Sized to the joist depth |
Openings for stairs and chimneys add trimmers and headers that this calculator handles only crudely, through the doubled joist field. A stair opening in a floor is a small structure in its own right: doubled trimmers each side, a doubled header at each end, and hangers at every one of those connections. If your floor has an opening bigger than a chimney chase, take it off separately.
Depth, span and the thing this will not tell you
Nothing here decides how deep the joist has to be. Depth comes from span tables that vary by species, grade, spacing, load and the edition of the model code your jurisdiction adopted and amended. Two towns fifty miles apart can land on different answers for the same floor. The building department that will inspect the work is the one whose table counts.
What is worth carrying in your head is the shape of the problem. Sawn joists get bouncy long before they get weak. A span-to-depth ratio past about 24 to 1 is where people start describing a floor as springy even though it is nowhere near failing, and tile and stone are much less forgiving of that deflection than carpet is. If the floor is going to carry tile, the deflection limit that applies is tighter than the one for a bedroom, and that is a conversation to have before the lumber is on the truck.
Once the joists are down, the sheets go on top: work the subfloor out on the plywood sheet calculator, and if the floor hangs off a beam, put the load through the header and beam load calculator before anyone picks a member size.
Permits
Structural floor framing is permit work almost everywhere, and it gets inspected before the subfloor covers it. That inspection is easier to pass than most people fear, because the inspector is mostly looking at bearing, hangers with every hole filled, and notching and boring that went too far. It is much harder to fix after the plumber has run a 3 inch drain through the middle of a joist.
Questions people ask
How many joists do I need for a 20 foot wall at 16 inches on center?
Sixteen. Twenty feet is 240 inches, 240 divided by 16 gives 15 spaces, and 15 spaces need 16 joists because there is one at each end of the run. Then add whatever the plan needs on top of the plain layout: a doubled joist under any partition wall that runs parallel above, trimmers each side of a stair or chimney opening, and the rim and band around the outside. A plain 20 by 14 floor with one partition doubled comes out at 17 joists plus four pieces of band and rim.
What length blocking do I cut for 16 inch on center joists?
Fourteen and a half inches. The 16 inches is measured center to center, and a nominal 2x joist is 1-1/2 inches thick, so the clear gap between two joists is 16 minus 1.5. At 24 inch centers it is 22-1/2, and at 12 inch centers it is 10-1/2. Cut a couple and check them in the actual bays before you cut a hundred, because framing lumber is not always where the layout says it is, and a blocking piece that is a sixteenth too long will push the joist off layout and put your subfloor seams in the wrong place.
Does this calculator tell me what size joist I need?
No, and it deliberately does not try. Joist depth depends on the span, the spacing, the species and grade of the lumber, the loads the floor carries and the deflection limit that applies to the finish going on top. Those combinations live in span tables, and which table applies is decided by the code your jurisdiction adopted and any local amendments to it. Use the table your building department works from, or have an engineer size it. A web page that gave you a depth without knowing your species, grade or jurisdiction would be guessing.
When do I need hangers instead of just bearing on the wall?
When the joist end has nothing under it. A joist sitting on top of a wall plate or a beam is bearing, and it needs no hanger. A joist framing into the side of a ledger board or into the face of a flush beam is hanging, and the hanger is the connection. Set the ends field accordingly. If you use hangers, fill every nail hole the manufacturer specifies with the fastener they specify — hanger capacity is published for a specific nail in a specific pattern, and a hanger with half its holes empty is a different and much weaker connection than the one on the label.
Can I notch or drill a joist to run plumbing through it?
Within limits that are much tighter than people assume, and the limits differ for notches and for holes. The general shape of the rules everywhere is that holes belong near the neutral axis, well away from the top and bottom edges, and notches at the ends are restricted to a small fraction of the depth while notches in the middle third of the span are not allowed at all. Because the exact fractions come out of the adopted code and can be amended locally, check the figures your inspector uses before the saw comes out. A 3 inch drain through the belly of a joist is not something you can patch afterwards.