How to use the shelf spacing calculator
- Enter the total height of the space in inches, from its bottom edge to its top edge.
- Enter the number of shelves and the shelf thickness.
- Choose how to count: Shelves between ends if the space is already closed top and bottom, or Ends count as shelves if your top and bottom boards are two of the shelves.
- Read the gap between shelves and the table of where each shelf's bottom and top edges go, measured up from the bottom.
Formula
Total thickness = shelves x thickness
Gaps = shelves + 1 (between ends) or shelves - 1 (ends count)
Gap = (height - total thickness) / gaps
The bottom edge of shelf k is k x gap + (k - 1) x thickness when the ends are not counted. The gap is also shown to the nearest 1/16 inch.
A 72 inch tall bookcase
Four shelves of 0.75 inch plywood fit in a 72 inch space. The total thickness is 4 x 0.75 = 3 inches, there are 5 gaps, and each gap is (72 - 3) / 5 = 13.8 inches, which is 13 13/16 inches to the nearest 1/16. The bottom edges sit at 13.8, 28.35, 42.9 and 57.45 inches.
If instead the top and bottom boards are two of the four shelves, there are 3 gaps of (72 - 3) / 3 = 23 inches, and the shelves start at 0, 23.75, 47.5 and 71.25 inches.
Even gaps, and when not to use them
Equal spacing is easy to build and looks tidy, but it is not always the best use of the height. A bookcase holding paperbacks, cookbooks and large art books might have gaps of different sizes. If you need unequal gaps, subtract the fixed ones first and divide the rest.
Cut a spacer block to the gap and rest each shelf on it while you fasten the supports. That is more accurate than measuring each shelf from the floor.
Strength and sag
The calculator only divides the height. It does not check whether a shelf is thick or stiff enough for its span and the weight it will carry, and long shelves sag under heavy loads. A thicker shelf, a center support or a front edge strip all reduce sag.
For related planning, see the picture hanging calculator, the board foot calculator, the stud calculator and the linear feet calculator.
Assumptions and limits
- All shelves are the same thickness and the gaps are equal.
- Heights are measured from the bottom edge of the space.
- The nearest 1/16 inch value is rounded for convenience and the decimal value is exact to the digits shown.
- Shelf strength, supports and the weight they carry are not modeled.
Frequently asked questions
How do I space shelves evenly?
Subtract the total thickness of the shelves from the height and divide by the number of gaps. For 4 shelves of 0.75 inch in 72 inches, that is 69 divided by 5, or 13.8 inches.
How many gaps are there for a given number of shelves?
If the space is closed at the top and bottom, there is one more gap than shelves. If the top and bottom boards are counted as shelves, there is one fewer gap than shelves.
How far apart should bookshelves be?
It depends on what they hold. Paperbacks need less height than large books. Measure the tallest item you plan to store and add a little clearance, then check that the equal gap is at least that.
What if the gap comes out between two tape measure marks?
The tool shows the gap to the nearest 1/16 inch. A cut spacer block set to that gap keeps all shelves consistent, and a small difference at the top is easier to hide than uneven gaps.