A sheathing calculator does one division: area divided by the area of a sheet, rounded up. What makes the number useful on site is the handful of things the division ignores. Openings are deducted or not depending on how you cut, bracing panels may be a different material, roof sheathing carries more waste than walls because of hips and valleys, and the count that gets ordered has to include the allowance you actually see returned rather than a figure copied from a guide.
Measure the area, then decide about openings
Wall area is perimeter by height; roof area is the plan area adjusted for pitch. Deduct only the large openings: sheathing is usually run past small ones and cut, because the cut-out is faster than the joint and the offcut often has a use.
Roofs waste more than walls
Hips, valleys and any cut-up geometry produce offcuts that cannot be used elsewhere. Set the allowance from what you returned on the last few roofs rather than from a rule of thumb, and write it on the estimate as a line so the customer sees it.
Order the count plus the fixings
Sheets are the headline and the order is not complete without nails or screws by weight, clips, tape or membrane where the specification calls for it, and the bracing panels if they differ. Those are the lines that send somebody to the merchant mid-morning.
Questions people ask about sheathing calculator
Should openings be deducted?
Deduct large ones such as garage doors and window walls; ignore standard windows and doors. Cutting from a full sheet is normally faster than seaming around the opening, and the offcuts are usable.
How much waste is normal?
Less on a plain rectangle and considerably more on a cut-up roof. The only number worth using is your own, taken from what came back from the last few jobs of the same shape.
Does thickness change the count?
No, but it changes the price, the fixing schedule and sometimes the span rating, which is a design decision from the drawings rather than an estimating one.