How many sheets of drywall do I need for a room?

Find the wall area (2 x (length + width) x height), subtract about 20 square feet per door and 15 square feet per window if you want to skip openings, add the ceiling (length x width) if you are boarding it, add 10 to 15 percent for cuts and waste, then divide by the area of one sheet and round up.

How many bags of concrete are in a cubic yard?

An 80 lb bag of standard premix makes about 0.60 cubic feet, so a cubic yard (27 cubic feet) takes about 45 bags. A 60 lb bag makes about 0.45 cubic feet, roughly 60 bags per cubic yard.

How do I work out the volume of a concrete slab?

Multiply length by width by thickness, with all three in the same unit. Divide cubic feet by 27 to get cubic yards. Then add 5 to 10 percent for uneven ground and spillage before you buy.

How many drywall screws per sheet?

A common rule of thumb is about one screw per square foot of board, which is roughly 32 screws for a 4 x 8 foot sheet.

Drywall Sheets and Concrete Bags Calculator (with waste)

Drywall (plasterboard) sheets

Concrete slab: premix bags

The drywall math, step by step

Everything is plain area arithmetic. For a 14 x 12 ft room with 8 ft walls, one door and two windows, boarding the walls only:

wall area      = 2 x (14 + 12) x 8        = 416 sq ft
openings       = 1 door x 20 + 2 windows x 15 = 50 sq ft
board area     = 416 - 50                 = 366 sq ft
with 10% waste = 366 x 1.10               = 402.6 sq ft
4 x 8 ft sheet = 32 sq ft
sheets         = 402.6 / 32 = 12.58       -> 13 sheets

Three things move the answer most. Sheet size: longer sheets mean fewer joints, but only if you can carry them into the room. Openings: many installers board straight over doors and windows and cut them out afterwards, which gives offcuts for small patches. If you do that, put 0 in the door and window boxes. Waste: 10 to 15 percent is a usual range for walls; a small or awkward room with lots of corners is the case for the high end.

Screws

The calculator also shows an approximate screw count: about one screw per square foot of board you are buying (around 32 for a 4 x 8 ft sheet). Treat it as a shopping estimate; your spacing may differ.

The concrete math, step by step

Volume is length x width x thickness in one unit. A 10 x 10 ft patio, 4 in thick:

thickness      = 4 / 12                   = 0.333 ft
volume         = 10 x 10 x 0.333          = 33.3 cu ft
with 10% waste = 33.3 x 1.10              = 36.7 cu ft  (1.36 cu yd)
80 lb bag      = 0.60 cu ft
bags           = 36.7 / 0.60 = 61.1       -> 62 bags

Bag yields are proportional to the weight of the bag. The calculator assumes that an 80 lb bag of standard premix gives 0.60 cu ft of placed concrete and scales from there. Brands differ a little, so look for the yield printed on your bag and, if it is different, treat the result as a close estimate.

Yield per bag, using 0.60 cu ft for 80 lb
Bag	Makes about	Bags per cu yd (or m³)

For footings, pad footings and round piers as well as slabs, use the concrete calculator.

Ready-mix trucks are sold by volume, not bags. If a job is much bigger than a few dozen bags, compare the cost of delivery with mixing by hand.

Common questions

Should I subtract doors and windows?

It is your call. Subtracting gives the tightest count. Boarding over openings and cutting them out later wastes more board but is quicker to hang. The calculator lets you pick either by changing the door and window boxes.

How much waste should I add to concrete?

5 to 10 percent is typical for a slab on prepared ground, because the base is rarely perfectly level and some mix is lost in mixing and finishing. Round up a little more if your forms are rough.

What thickness is a patio slab?

The calculator starts at 4 inches (10 cm), which is a common figure for light foot traffic. For driveways or anything with vehicles, follow local building guidance for thickness and base.

Does this account for rebar, gravel base or formwork?

No. It calculates the concrete or board only. Gravel, mesh and lumber are separate quantities.