What this calculator works out
This calculator takes an internal height and a number of shelves, subtracts the thickness of the shelves themselves, and divides the remainder into equal clear openings.
Subtracting the shelf thickness is the step people forget. Five 18 mm shelves take up 90 mm of height before anything is stored on them.
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Before you set out
- Internal height means the clear space inside the unit, not the overall external height.
- Shelf thickness is in millimetres. 18 mm is standard for MDF and chipboard; solid timber and glass differ.
- Equal spacing is rarely what you want. Work out what goes on each shelf first, then set heights to suit.
- Allow finger room above the tallest item on each shelf — 20 to 30 mm makes things far easier to lift out.
Paperbacks need about 20 cm of clear height, hardbacks 25 to 30 cm, LP records 32 cm, box files 32 cm and A4 lever arch files about 32 cm on end. Designing shelves to those figures rather than to equal spacing usually gets more onto the same unit.
How this calculator works
Height less shelves, divided into openings:
Clear opening = (internal height − (shelves × thickness ÷ 10)) ÷ (shelves + 1)
Thickness is converted from millimetres to centimetresThe number of openings is one more than the number of shelves, because the space above the top shelf and below the bottom one both count. Five shelves create six spaces.
Worked example: 180 cm with five shelves
Using the default figures — 180 cm internal height, 5 shelves at 18 mm:
- Shelf thickness total: 5 × 1.8 = 9 cm
- Remaining height: 171 cm
- Clear opening: 28.5 cm across 6 spaces
28.5 cm takes hardbacks, box files and LPs, which makes it a genuinely useful general-purpose spacing. Dropping to four shelves gives 34.6 cm across five spaces — better for taller items and worse for books. Equal spacing is a reasonable starting point, but the better design usually puts one tall opening at the bottom and tighter ones above.
Common mistakes
- Forgetting the shelf thickness. Five shelves can absorb 9 cm or more.
- Counting spaces as equal to shelves. There is always one more space than shelves.
- Using external height. The top and bottom panels reduce the internal space.
- Spacing everything equally. Mixed heights fit more in.
- Ignoring shelf sag. Long unsupported spans bow under load, particularly with books.
Frequently asked questions
How far can a shelf span without sagging?
For 18 mm MDF or chipboard carrying books, around 700 to 800 mm between supports is a practical maximum before visible sag develops over time. Solid timber and plywood do better; glass and melamine-faced chipboard do worse. Adding a batten along the back edge, or a lipping along the front, stiffens a shelf substantially.
What is a comfortable height for the top shelf?
Anything above about 180 cm needs reaching or a step for most people, and above 200 cm is awkward for everyone. Put things you use often between roughly 60 and 150 cm, and reserve the highest shelf for light, rarely needed items.
Should shelves be fixed or adjustable?
Adjustable is more flexible and only slightly more work — a row of holes at 32 mm centres, which is the standard cabinet-making increment, or metal shelf strips. Fixed shelves are stronger and better where the shelf contributes to the unit's rigidity. Many units use fixed shelves at top, bottom and middle with adjustable ones between.
Do tall units need fixing to the wall?
Yes. Any unit tall enough to tip is a hazard, particularly with children in the house, and the risk increases as upper shelves are loaded. Most flat-pack units include a bracket for this; use it, and fix into a stud or with an appropriate masonry fixing rather than into plasterboard alone.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — product safety guidance, including furniture tip-over risk
- Health and Safety Executive — safe storage and racking stability
Sources are checked at publication and can change — how I choose and check references.