Shelf Spacing Calculator
Space shelves evenly inside a carcass and get the exact position of every board, measured from the inside bottom. Tell it how many physical shelf boards you are installing — the compartment count follows from that, and both are shown together.
Calculator
Between the inside faces of the top and bottom panels — not the outside height.
Boards you cut and install. 0 is valid.
Result
Each clear opening
345.6 mm
4 shelf boards → 5 compartments
Board positions, measured up from the inside bottom:
- Board 1underside 345.6 mm · top 363.6 mm · centre 354.6 mm
- Board 2underside 709.2 mm · top 727.2 mm · centre 718.2 mm
- Board 3underside 1,072.8 mm · top 1,090.8 mm · centre 1,081.8 mm
- Board 4underside 1,436.4 mm · top 1,454.4 mm · centre 1,445.4 mm
Positions assume fixed shelves of equal thickness housed in the carcass, measured from the inside bottom. It does not check whether a shelf will sag: span, thickness, material and load all matter, and this calculator makes no structural claim.
Boards or openings? Say which
"I want four shelves" is ambiguous, and the ambiguity costs a board. Someone picturing a bookcase with four shelves might mean four physical boards to cut and install — which creates five openings — or four usable openings, which needs only three boards.
Get it wrong and every opening height is wrong too, because the available height is divided by the wrong number. This calculator never asks for "shelves". It asks for the number of physical internal shelf boards, and reports the resulting compartment count alongside it, so the two can never be confused.
The relationship is always the same: compartments = boards + 1. Zero internal boards is a legitimate answer and gives you one full-height compartment.
The formula
Available opening height = internal clear height − (number of boards × board thickness).
Compartment count = number of boards + 1.
Equal opening height = available opening height ÷ compartment count.
Board thickness has to come out first. Each board occupies its own thickness of the carcass, so four 18 mm shelves consume 72 mm before any opening is measured. Dividing the raw internal height by the compartment count — a common shortcut — gives openings that are each a fraction of a board too tall, and the error accumulates up the carcass until the top shelf does not land where the drawing says.
Worked example
A bookcase has 1800 mm of clear internal height and you want four 18 mm shelf boards inside it.
Board thickness consumed: 4 × 18 = 72 mm. Available opening height: 1800 − 72 = 1728 mm. Compartments: 4 + 1 = 5. Equal opening: 1728 ÷ 5 = 345.6 mm.
So the first board sits with its underside 345.6 mm above the inside bottom and its top face at 363.6 mm. The second board's underside is at 709.2 mm, and so on. The last board's top face is at 1454.4 mm, leaving a final 345.6 mm opening to the underside of the top — which confirms the arithmetic closes.
Where the measurements are taken from
Every position is measured from the inside bottom of the carcass — the top face of the bottom panel — because that is the surface you can actually register a rule against when marking out.
Board positions and clear openings are different things and both are given. The board bottom and board top bracket the material; the opening is the gap between one board's top face and the next board's underside. When you are marking dados, the centreline is usually the more convenient reference, so that is provided too.
If you are cutting dados, remember that a dado adds to the shelf length rather than changing these positions: the shelf sits into the side panels, so the board is longer than the internal width by twice the dado depth. That is a cut-list concern, not a spacing one.
Common mistakes
Ignoring board thickness in the division, as above. It is the single most common shelf-layout error and it always makes the openings slightly too tall.
Spacing everything equally when the contents are not equal. Paperbacks need about 200-230 mm of clear height, hardbacks 250-300 mm, LPs about 320 mm, and A4 files around 320 mm. A perfectly even bookcase often wastes height at the top and cannot take a large book anywhere. Set a custom bottom clearance for the tall things and let the rest divide equally.
Forgetting that a very long unsupported shelf will sag under load regardless of how well it is positioned. Spacing is a layout question; sag is a span and material question this tool does not answer.
Get the whole bookcase, not just the spacing
Shelf positions are one line of a cut list. Describe the bookcase and Plank CAD produces every part, with dado allowances already worked into the shelf lengths, a structural check and exports.
Common questions
It depends on what goes on them. Around 200-230 mm of clear height suits paperbacks, 250-300 mm suits most hardbacks, and 320 mm or more is needed for LPs, A4 box files and large art books. Many bookcases use a taller bottom compartment for oversized items and divide the remaining height equally.
Five physical internal boards create six compartments. The count is always boards plus one, because the carcass bottom and top each form one side of a compartment without being an internal board themselves.
Inside. Enter the clear internal height between the inside faces of the top and bottom panels, and all positions returned are measured up from the inside bottom. Using external height would include the panel thicknesses and place every shelf too high.
No, and it deliberately does not guess. Deflection depends on span, thickness, material stiffness and the load, and a wrong reassurance about a shelf holding books would be worse than no answer. As a rough guide, 18 mm plywood spans up to roughly 750-800 mm under a normal book load, but verify against your own material.