How is equal shelf spacing calculated?
Subtract the total thickness of all shelves from the inside opening height. Divide what remains by the number of shelves plus one. That creates an equal clear gap below the first shelf, between shelves, and above the last shelf.
Clear gap = (opening height − shelf count × shelf thickness) ÷ (shelf count + 1). The bottom edge of shelf i, measured upward from the opening bottom, is i × clear gap + (i − 1) × thickness.
Worked example
A 36-inch opening with three ¾-inch shelves leaves 33.75 inches clear. Four equal spaces are each 8.4375 inches. Mark the shelf bottoms at 8 7/16, 17 5/8 and 26 13/16 inches (8.4375, 17.625 and 26.8125) from the opening bottom.
Where do the shelves go in common cabinet openings?
The table below is computed by this calculator for 3/4-inch shelves. Marks are measured from the bottom of the opening to the underside of each shelf, rounded to the nearest 1/16 inch (≈ marks a rounded value).
| Inside opening (in) | Shelves | Clear gap (in) | Shelf-bottom marks from the bottom (in) |
|---|---|---|---|
| 30 | 2 | 9 1/2 | 9 1/2 · 19 3/4 |
| 36 | 2 | 11 1/2 | 11 1/2 · 23 3/4 |
| 36 | 3 | 8 7/16 | 8 7/16 · 17 5/8 · 26 13/16 |
| 42 | 3 | 9 15/16 | 9 15/16 · 20 5/8 · 31 5/16 |
| 72 | 4 | ≈ 13 13/16 | ≈ 13 13/16 · ≈ 28 3/8 · ≈ 42 7/8 · ≈ 57 7/16 |
| 72 | 5 | 11 3/8 | 11 3/8 · 23 1/2 · 35 5/8 · 47 3/4 · 59 7/8 |
| 84 | 6 | ≈ 11 3/8 | ≈ 11 3/8 · ≈ 23 7/16 · ≈ 35 9/16 · ≈ 47 11/16 · ≈ 59 13/16 · ≈ 71 7/8 |
Does real plywood thickness change the marks?
Barely. Plywood sold as 3/4 inch is often thinner: a sanded 23/32 Performance Category panel may measure 0.703 to 0.734 inch under PS 1-22. Re-running every row of the table with 0.703-inch shelves moves no mark by more than 0.040 inch — less than one 1/16-inch graduation on the tape — because thinner shelves both widen the gaps and take up less height. Enter the measured thickness anyway; it matters more for the dado or shelf-pin fit than for the marks.
How do I transfer the marks accurately?
- Measure the opening at both sides; if the two differ, the cabinet is out of square — fix that before marking.
- Mark every shelf from the same reference, the bottom of the opening, instead of measuring gap after gap, so small errors do not add up.
- Transfer the marks to a story stick and use it on both sides of the cabinet so the shelves stay level.
What does this estimate leave out?
This is layout geometry, not a structural shelf-span or load rating. It assumes a level opening and shelves of uniform thickness. It does not reserve clearance for adjustable shelf pins, door hinges or face frames. Measure the actual opening and check hardware before cutting.
Frequently asked questions
Can I type fractions?
Yes. In inches the fields accept decimals (0.75), simple fractions (3/4) and mixed numbers (35 1/2). Results are shown to the nearest 1/16 inch, marked “≈” when rounded, with the exact decimal alongside.
Should I measure the opening or the cabinet?
The inside opening: from the top of the bottom panel to the underside of the top panel. The outside height includes the panels themselves and would push every mark too high.
What thickness should I enter for 3/4-inch plywood?
Measure it. PS 1-22 allows sanded plywood in the 23/32 Performance Category, usually sold as 3/4 inch, to measure 0.703 to 0.734 inch. For the marks it barely matters; for a dado it does.
Does it work for adjustable shelves?
It gives the ideal equal positions. With shelf pins, move each shelf to the nearest hole: on 32 mm-system cabinets the holes are 32 mm apart, so the gaps become close to equal rather than exactly equal.
Does it check whether a shelf will sag?
No. This is layout geometry only, not a span or load rating.
Sources
- Plywood thickness by Performance Category: NIST, Voluntary Product Standard PS 1-22, Structural Plywood (Table 10).
- Exact inch: NIST — SI units of length (1 in = 25.4 mm).
Need to estimate sheet yield too? Use the Plywood Panel Calculator.