Drywall patch calculator.

The wrong way to compute a patch is to scale the room-size math down. A 1 square foot hole does not need a sheet of drywall, a 4.5 gallon pail of joint compound, a 250 foot roll of paper tape, and a pound of screws. That is what a linear scale-down gives you, and it is the reason most drywall patch calculators on the web are worthless. The fix is to route by method.

Mesh patch kit. Under a fist (roughly 0.25 square feet, a 6 by 6 inch hole or smaller). The fix is a self-adhesive aluminum mesh square with premixed compound in the same kit. $6 to $12 at any hardware store. Three coats of mud, sand, paint. Done in an afternoon. No sheet, no tape, no pail.

California patch. Fist to dinner plate (0.25 to 4 square feet). Also called a butterfly patch. Cut a scrap drywall piece slightly larger than the hole, score and peel the gypsum off the back to leave a paper flap on all four sides, slide the gypsum plug into the hole, glue the paper flap to the wall surface with mud. It floats on the paper, no backer strips, no framing. A 1 quart compound tub, a 75 foot mini tape roll, and a handful of screws are the entire shopping list.

Cut to studs. Bigger than 4 square feet. Cut back to the nearest studs on both sides of the opening, screw 1 by 3 pine furring strips across the existing studs as backers for the new piece, cut a fresh piece from a sheet, land it on the backers, tape and mud the four seams. Sheet math applies here. Compound and tape constants are different from the full-room calculator because a patch is almost entirely perimeter, not field.

Thickness is the first match. Old houses run a mix of 3/8 inch and 1/2 inch on 16-inch on-center framing, and the patch has to match or the plane of the wall breaks at the seam. Measure the existing board with a tape before you buy. If the old wall is 5/8 inch and you patch with 1/2 inch, you will need to shim the patch out 1/8 inch and skim-coat the edges, which is more work than buying the right thickness.

Texture is the second match. A smooth-wall patch on an orange-peel or knockdown finish will stand out forever unless you re-texture the patch zone plus a feathered halo around it. Rattle-can spray texture in a matching profile works for small areas. For anything larger than a 2×2 patch, it usually looks better to re-skim the whole wall to Level 5 than to chase a texture match.

Finish level is the third match. A Level 4 patch (three coats, sanded, primed) under a semi-gloss paint will show under raking light no matter how good your taping is, because the original wall has a different sheen where the factory-papered face is undisturbed. If the wall is semi-gloss or has a sconce that rakes across it, plan for Level 5 from the start.

The patch calculator asks one question that most web calculators skip: is the patch on a dwelling-garage common wall or garage ceiling. The rules from IRC R302.6 are specific and often misstated. On the garage side of a common wall, Table R302.6 requires at least 1/2 inch gypsum board or equivalent. That is the minimum, and 1/2 inch regular drywall meets it. 5/8 inch Type X is not required on an ordinary garage-to-dwelling common wall.

The 5/8 inch Type X rule kicks in when there is a habitable room above the garage. Then the garage ceiling that forms the floor-ceiling assembly for the room must be 5/8 inch Type X (or the listed equivalent), and any wall that structurally supports that assembly gets the same treatment. This is the case that catches people out: a simple detached-wall garage patch is fine at 1/2 inch, but a patch on a garage ceiling under a bedroom is not.

The warning this calculator fires when you check the garage box is intentionally conservative: it reminds you to verify which case you are in before you buy board. If the existing wall or ceiling is already 5/8 Type X, match it. If it is 1/2 inch and there is no room above, 1/2 inch is compliant per Table R302.6. When in doubt, check the full text at codes.iccsafe.org, IRC 2021 Section R302.6 and Table R302.6.

There is a point where patching stops being cheaper than re-doing the whole wall. Roughly, if you are replacing more than 25 percent of a wall's area, a full skim coat of the entire wall is less total work and always looks better. Two reasons. First, you only have to feather the compound once, into the baseboard and ceiling, instead of feathering three or four separate patch halos. Second, the finish plane ends up continuous, so no single patch shows under light.

For a bathroom rebuild or a water-damaged wall, the crossover is often 15 to 20 percent because bathrooms are small enough that re-skimming is fast and the light is unforgiving. For a garage or basement, the crossover is closer to 40 percent because nobody stares at a garage wall under a raking sconce.

If you are above the crossover, switch to the drywall calculator and treat the whole wall as the project. The ancillary math (mud, tape, screws) is the same, but the sheet count and the scope of the job change.