For a cross wrap, start with 2 × length + 2 × width + 4 × height, then add ribbon for your bow. Keep all measurements in the same unit.
Why this formula works
One band goes around the length and height of the box: 2L + 2H. The other goes around the width and height: 2W + 2H. Adding them gives 2L + 2W + 4H.
This is a geometric estimate for a rectangular box. It does not measure the ribbon taken up by a knot, folds, or the bow.
A 10 × 8 × 4 inch box
The wrapping paths alone use 20 + 16 + 16 = 52 inches.
If you allow 20 inches for a small bow, and add 5% for trimming, the estimate is (52 + 20) × 1.05 = 75.6 inches, or 2.1 yards.
The 20-inch bow allowance and 5% margin are working assumptions, not measured craft test results. A wider ribbon, larger loops, or long tails can use substantially more. Use a spare piece of ribbon or string to check your planned bow before cutting an expensive roll.
One band instead of a cross
For a band running along the length, use 2L + 2H. For a band running across the width, use 2W + 2H. Add your knot or bow allowance afterward.
Buying for several boxes
Multiply your estimated length per box by the number of boxes. Round up to a full spool, rather than rounding down your cut length.
For ten boxes at 2.1 yards each, the estimate is 21 yards. A 25-yard spool leaves 4 yards of spare ribbon under these assumptions. It does not guarantee ten finished bows of every size.
Use the ribbon length calculator to change the box dimensions, or the bow calculator to try different loop and tail lengths.