How to Estimate a Full Room Renovation

The mistake that costs you a second trip to the hardware store

I renovated my 1920s bungalow one room at a time over about three years. Bedroom first, then the kitchen, then the living room, and finally the bathroom that had been held together by optimism and caulk since the Nixon administration. By the time I got to that bathroom, I had the process dialled in. But early on? I made every rookie mistake in the book.

The worst one — and I see other people make it constantly — is guessing at material quantities. You eyeball the walls and throw six sheets of drywall in the truck. You grab two gallons of paint because the room “isn’t that big.” You order flooring based on the square footage number you half-remember from the listing when you bought the place. Then you run short, make a second trip, and discover that the next batch of flooring is from a different production run with a slightly different colour. Now you’ve got a visible seam in the middle of your hallway.

This guide is the process I wish someone had handed me before that first bedroom. We’ll work through a full room renovation in order — drywall, paint, and flooring — with a calculator for each stage so you get real numbers instead of guesses.

Start with the walls: drywall

If you’re doing a full renovation, there’s a good chance some or all of the drywall needs replacing. Old plaster walls in pre-war houses are notorious for hidden damage — pull off one section of wallpaper and suddenly you’re staring at crumbling lath. Even in newer homes, water damage or previous bad patch jobs can mean it’s faster to tear out and start fresh.

Here’s what you need to know before ordering:

• Measure every wall surface that needs new board. Height times width for each wall, then add them together.
• Subtract door and window openings — but only the openings, not the trim area. You’ll be cutting drywall around those openings and the offcuts usually end up in the skip.
• Pick the right thickness: 12.5mm (half-inch) for most walls, 15mm (five-eighths) for ceilings and anywhere you want extra fire resistance or sound dampening.
• Standard sheet sizes are 1200 x 2400mm (4 x 8 feet). Larger sheets mean fewer joints to tape, but they’re heavy and awkward in tight spaces.

A common trap is forgetting to account for waste from cuts. You’ll lose material at every corner, every outlet cutout, and every window frame. Budget at least 10% extra — more if the room has an unusual shape or lots of openings.

Use the Drywall Calculator to get your sheet count right:

Once you have your sheet count, order your joint compound and tape at the same time. For every 30 square metres (roughly 320 square feet) of drywall, expect to use about 15 kilograms of compound for three coats. Buy paper tape, not mesh — paper tape is stronger at corners and less likely to crack down the line.

Paint: the part everyone underestimates

After the drywall is hung, taped, and sanded, it’s time for paint. This is where most DIYers make their second big mistake: underbuying. Two coats over fresh drywall eats paint. The paper facing absorbs the first coat like a sponge, which is why a dedicated primer coat (or a high-quality paint-and-primer product) is non-negotiable on new board.

Here’s the formula: one litre of paint covers roughly 10 to 12 square metres per coat on a smooth surface. On fresh drywall, expect closer to 8 square metres for the primer coat. Textured surfaces use even more.

Things people forget to include in their paint calculation:

• Ceilings — if you’re painting them (and you should if the room has new drywall), that’s the entire floor area added to your wall area.
• Closet interiors — they’re part of the room and they need paint.
• Two coats minimum for the topcoat, on top of your primer. Three coats if you’re going from a dark colour to a light one.

Let the Paint Calculator give you a proper quantity:

Buy all your paint at once, from the same batch. Even the same colour formula can shift slightly between batches. If you need six tins, buy seven. Leftover paint stores well in a cool place and is useful for touch-ups down the road.

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Flooring: hard surfaces

With the walls finished, you can move to the floor. Whether you’re laying hardwood, laminate, vinyl plank, or engineered timber, the estimation process is similar — but the waste factors are different.

Key considerations:

• Room shape matters: a simple rectangle wastes less than an L-shaped room or a room with alcoves, bay windows, or angled walls.
• Plank direction: running planks lengthwise in a narrow room looks better but can generate more waste at the ends if the room length doesn’t divide evenly by the plank length.
• Stagger requirements: most manufacturers require at least a 300mm (12 inch) stagger between end joints in adjacent rows. Short offcuts from one row can’t always start the next row.
• Waste allowance: 10% for a straightforward rectangular room, 15% for complex layouts, and up to 20% if you’re running planks at a 45-degree angle.

One thing that catches people off guard is the difference between the nominal coverage on the box and the actual usable coverage. A box that says it covers 2 square metres assumes zero waste. In reality, you’ll use a bit more.

Run your numbers through the Flooring Calculator before you order:

Order all your flooring from the same batch — same advice as paint, same reason. Production runs vary, and a colour mismatch in the middle of your floor is painfully obvious.

Carpet: the soft option

If you’re carpeting instead of (or in addition to) hard flooring, the estimation is a bit different. Carpet comes in rolls of a fixed width — usually 3.66m (12 feet) or 4m (roughly 13 feet) — and you need to plan your seam placement carefully.

Things to account for:

• Roll width versus room width: if your room is wider than the roll, you’ll need a seam. Place seams in low-traffic areas and away from doorways where they’ll get kicked.
• Pile direction: all pieces must run the same way, or the carpet will look like two different colours where the seams meet. This limits how you can rotate offcuts.
• Pattern matching: patterned carpet needs extra material to align the pattern at seams, sometimes 10-15% more.
• Doorways and closets: these small areas still consume full-width carpet. A 1m x 0.6m closet floor still comes off a 3.66m-wide roll.

Use the Carpet Calculator to figure out how much you need:

Don’t forget underlay. Most carpet installations need it, and it’s sold by the square metre. Match your underlay to the carpet type — foam underlay for most domestic carpets, rubber or felt for heavy-duty options.

Pull it all together: your renovation checklist

Before you load up the truck or place your delivery order, run through this list:

• Drywall: sheets, joint compound, paper tape, screws (roughly 30 screws per sheet)
• Paint: primer, topcoat, rollers, tray liners, painter’s tape for trim and edges
• Flooring: planks or tiles, underlay or moisture barrier, spacers, transition strips for doorways
• Carpet: carpet, underlay, gripper rods, seam tape, knee kicker rental

The calculators above give you the core material quantities. The ancillary items — screws, tape, spacers — are cheap, so buy more than you think you need. Running out of painter’s tape mid-wall is annoying. Running out of drywall screws when the hardware store is closed is worse.

One last piece of advice: do not start demolition until every material is on site or confirmed for delivery. I learned this the hard way when I ripped out a bedroom floor on a Friday, expecting my flooring delivery on Monday. The delivery got delayed a week. I spent seven days stepping over joists in my socks. Don’t be that person. Get your materials sorted first, then start swinging the hammer.

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