A free browser tool
Your wall, your cut list, and a shelf that will not sag.
Plans are sold at one fixed size, and the sag calculator is a different tool that knows nothing about your plan. This does both: the cut list for your dimensions, checked against the load you intend to put on it.
Step 1
The bookcase
Step 2
Your plan
Did you get the result you wanted?
How it works
The cut list and the span check, from the same numbers
- Enter the wall you are filling and what will sit on the shelves.
- The span is checked with the standard beam-deflection formula, including the sag wood adds over the years.
- If it fails you get the fixes in order of cost, not just a warning. Then print the elevation and the cut list.
FAQ
Before you cut anything
Where does the sag number come from?
The deflection of a uniformly loaded beam supported at both ends, with published stiffness values per material. Wood creeps, so the figure includes the extra half again that a shelf picks up over time. The accepted limit is 0.02 inches of droop per foot of span.
How exact is it?
Stiffness values are laboratory averages for a species, and a real board varies. Treat a result close to the limit as a fail, and buy the thicker stock.
Why suggest a divider first?
Sag rises with the fourth power of the span, so halving the span is worth far more than any change of material. It is also the cheapest thing to add.
Is this free?
Yes. This validation version has no payment or account.
Is what I type stored?
No. Everything is worked out and drawn in this tab. Anonymous usage events and feedback are recorded.
Privacy
We record anonymous interaction events, a random browser session identifier, referral source, device category, and feedback. We do not intentionally collect names, email addresses, IP addresses, or the text entered into the tool.
Terms
This experimental tool is provided as-is. The span check is a guide from published formulas, not a structural guarantee. Measure your own stock before cutting.