Tile Spacing Calculator: Make the Run Come Out Even
Sometimes the fix for an ugly sliver at the end of a run is not a different tile — it is a joint width a sixteenth of an inch different. This works out which.
Your run
What to buy
- Full tiles that fit — tiles
- Leftover to cut — If this is under about 2 in, it will look like a mistake.
- Leftover as a share of a tile — %
- Grout joints in the run — joints
- Joint width for an exact fit — Same tile count, no cut at all — if this lands inside your tolerance.
- Is that within tolerance? — 1 = yes, 0 = no
- Joint width for one more tile — Tighter joints, one extra tile, still no cut. Negative means it cannot be done.
- Is that within tolerance? — 1 = yes, 0 = no
The joint is an adjustable dimension
Most people treat the grout joint as fixed — you buy 1/8 inch spacers, you use 1/8 inch joints — and then accept whatever cut lands at the end of the run.
Tile setters treat it the other way round. The joint has a workable range rather than a single correct value, and moving within that range is the cheapest way to make a run come out even. Across twenty joints, going from 1/8 to 3/16 of an inch buys you an inch and a quarter of run. That is often exactly the difference between a full tile at the end and a sliver.
This calculator works out how many tiles fit at the joint you want, how big the leftover is, and what joint width would make the run land exactly — with the tile count you have, and with one more.
What it is doing
module = tile + joint
full tiles = (run + joint) ÷ module, rounded down
length used = full tiles × module − joint (no joint at the ends)
leftover = run − length used
joint for an exact fit with n tiles = (run − n × tile) ÷ (n − 1)
joint for an exact fit with n + 1 tiles = (run − (n+1) × tile) ÷ n
The run is assumed to have tile hard against the wall at both ends, with joints only between tiles. If you are leaving a caulked expansion gap at the perimeter — and on a long run or over a heated slab you should — take it off the run length before you start.
A negative answer for the tighter option means it cannot be done: the tiles alone are already wider than the run.
When to use it
Niches. This is where it earns its keep. A shower niche is a short, highly visible run with hard dimensions at both ends, and a full tile at each end reads as craftsmanship in a way nothing else in the room does. Work the niche out first and let the surrounding field follow.
Backsplashes. A 14 inch run between a window casing and a cabinet, in 3 × 6 subway tile, will either land beautifully or leave you a half-inch strip against the casing. Two sixteenths of joint adjustment decides which.
Stair risers, hearths, thresholds, window returns. Anything short, visible, and bounded at both ends.
Long floors, less so. Over thirty feet the accumulated variation in the tiles themselves is bigger than the adjustment you are making.
The limits of the trick
Three honest caveats.
Non-standard joint widths mean no spacers. Spacers come in 1/16, 1/8, 3/16, 1/4. If the exact-fit answer is 0.153 inches, you are eyeballing every joint, which is fine across eight tiles and unworkable across eighty. Judge the answer against the run length, not just against your tolerance range.
Tiles are not the size on the box. Pressed, non-rectified tile varies from piece to piece by more than people expect, and over a long run that variation swamps a sixteenth-inch adjustment. Measure five tiles from different boxes and use the average. Rectified porcelain is mechanically ground after firing and is far more consistent — that is what you are paying for.
Never vary the joint along the run to absorb error. Starting at 1/8 and drifting to 3/16 by the far wall is instantly visible, especially on a wall where the eye has a straight line to compare against. Pick a width and hold it.
When neither exact fit works
Then you cut, and the only question is where.
Do not put a single narrow cut at one end. Take one full tile out of the run and split the remainder between both ends. Two cuts of six inches read as deliberate; one cut of three quarters of an inch reads as running out of wall.
That is the same logic the layout planner applies across a whole room, in both directions at once.
Sanded, unsanded, and the tolerance you set
The width you can move to is bounded by the grout as well as by looks. The traditional threshold is 1/8 inch: sanded above, unsanded below, because unsanded shrinks and cracks in a wide joint while sanded will not pack into a narrow one.
So a "1/16 to 3/16" tolerance range spans two grout types. If you have already bought the grout, set your tolerance to stay on one side of the line — or check the bag, since many modern grouts are rated across the whole range and make the question moot.
Tile manufacturers also specify a minimum joint for their product, and it is not decoration. Pressed tile with any size variation needs a joint wide enough to absorb it. Setting a 1/16 inch joint on tile that wants 1/8 gives you lippage and a wandering grout line.
A worked example
An 8 foot run — 96 inches — in 12 inch tile with 1/8 inch joints.
- Module: 12.125 inches
- Full tiles: (96 + 0.125) ÷ 12.125 = 7.93 → 7 full tiles
- Length used: 7 × 12.125 − 0.125 = 84.75 in
- Leftover: 96 − 84.75 = 11¼ inches, which is 94% of a tile
That is a fine outcome — a 11¼ inch cut looks essentially like a full tile. But look at what an exact fit would take:
- With 7 tiles: (96 − 84) ÷ 6 = 2 inch joints. Absurd.
- With 8 tiles: (96 − 96) ÷ 7 = zero joint. Also impossible.
Eight 12 inch tiles is exactly 96 inches, so the run is a pathological case: it is one tile away from perfect in both directions and cannot get there. The right move is the one the numbers already gave you — 7 full tiles and a fat 11¼ inch cut, placed where it shows least.
Change the run to 8 feet 2 inches and the picture changes completely: 8 tiles at a joint of (98 − 96) ÷ 7 = 2/7 inch, or a shade over 1/4. Wide, but real — and a run of eight full tiles with no cut at all.
What this calculator assumes
- The run is assumed to have no grout joint at either end — tile against the wall at both ends, with joints only between tiles. If you are leaving a caulked expansion gap at the perimeter, subtract it from the run before you start.
- Exact-fit joint widths are the mathematical answer, not necessarily the practical one. A joint width that is not a standard spacer size means eyeballing every joint, which works over eight tiles and does not over eighty.
- Tiles are assumed to be exactly the size you entered. Non-rectified tile varies by a surprising amount from piece to piece, and on a long run that variation is bigger than the adjustment you are trying to make. Measure five tiles and use the average.
- This looks at one run in one direction. A floor has two, and they rarely both come out well — that is what the layout planner is for.
Common questions
Is it cheating to change the joint width to avoid a cut?
No, it is the oldest trick in tiling. Grout joints have a workable range rather than a single correct value, and moving from 1/8 to 3/16 across twenty joints buys you an inch and a quarter of run. What you cannot do is vary the joint along the run to absorb the error — the eye picks that up immediately, especially on a wall.
What if neither exact fit is within tolerance?
Then you cut, and the question becomes where. On a wall or a floor, split the difference: take one full tile out and put half the remainder at each end. Two cuts of six inches look intentional; one cut of three quarters of an inch looks like you ran out of wall.
Does this work for a niche?
It is where it is most useful. A shower niche is a short, highly visible run with a hard dimension at both ends, and it is the one place where a full tile at each end genuinely reads as craftsmanship. Work the niche out first and let the field around it follow.
Why is my measured run different at the top and the bottom?
Because walls are not parallel. Measure at both ends and at the middle, use the smallest figure for planning full tiles, and let the perimeter cuts absorb the difference — that is what they are for.
Where these numbers come from
- The arithmetic is shown in full on the page. There is no external standard for joint width beyond the tile manufacturer's recommendation and the sanded/unsanded threshold.
Last checked 2026-08-11.