New Bathroom Calculators

A bathroom refit packs more trades into fewer square metres than any other room: plumbing, electrics, ventilation, waterproofing, tiling and decoration all overlap, and most of the expensive mistakes are order-of-work mistakes. This page is a working template for the whole job: the task sequence, what you will be ordering, where the regulations touch the work, and the calculator that serves each step.

The sequence that matters most: everything that lives inside the walls and floor (pipes, cables, fan ducting) goes in at first fix, the wet areas are waterproofed before tiling, and silicone sealing is one of the last jobs rather than the first. Get those three right and the rest of the job is quantities, which is what the calculators below are for.

Work in this order

1. 1

   Plan the layout before buying anything

   Check the room actually fits the suite you want: activity space around each fitting with the Bathroom Spacing Calculator, compact layouts with the En-Suite Layout Calculator, and enclosure sizes with the Shower Enclosure Calculator. Check the floor build-up against the door threshold with the Bathroom Underfloor Prep Calculator before committing to tiles and heating mats.

2. 2

   Strip out and first fix plumbing

   With the room stripped, hot and cold supplies and waste runs are moved to the new positions. Check supply pipe sizes with the Pipe Sizing Calculator, whether the shower needs a pump with the Shower Pump Calculator, and whether the cylinder and boiler can serve the new demand with the Hot Water Cylinder and Boiler Size calculators.

3. 3

   First fix electrics, respecting the zones

   Bathroom electrics are location-sensitive: what can be fitted where depends on its distance from the bath or shower, per the zone rules in BS 7671. Check positions with the Bathroom Electrical Zone Calculator, size any shower circuit with the Cable Sizing Calculator, and check what is notifiable with the Part P Notification Calculator. Cables go in now, while the walls are open.

4. 4

   Ventilation ducted in at first fix

   A bathroom needs mechanical extraction, and the fan and its duct route are far easier to install before the ceiling and walls are closed up. Size the fan with the Bathroom Extraction Rate Calculator and the ducting with the Bathroom Ventilation Calculator.

5. 5

   Board the walls and floor for wet service

   Wet areas need a substrate that suits tiling: moisture-resistant plasterboard or, in showers and wet rooms, tile backer board. Quantify boards with the Plasterboard Calculator and Tile Backer Board Calculator, and level the floor first if it needs it with the Self-Levelling Compound Calculator.

6. 6

   Waterproof before a single tile goes on

   Tanking comes before tiling in wet areas: tiles and grout alone are not a waterproof layer. Quantify the membrane and accessories with the Wet Room Tanking Calculator or Tile Waterproofing Calculator, and get the floor falls right with the Shower Tray Fall Calculator or, for wet rooms, the Wet Room Drainage Calculator.

7. 7

   Underfloor heating, then tiling

   Electric mats go down after the floor prep and before the tiles: size them with the Heated Bathroom Floor Calculator. Then tile: quantities from the Wall Tile and Floor Tile calculators, with the Tile Adhesive, Grout and Tile Trim calculators for the consumables.

8. 8

   Second fix, seal, decorate

   Sanitaryware, brassware, the fan and any towel rail go in at second fix: size the rail with the Towel Rail Sizing Calculator. Silicone sealing of joints and junctions is one of the last jobs, after grouting, and needs its cure time before the shower or bath is used. Decorate non-tiled surfaces with a suitable paint: the Anti-Mould Paint Calculator covers the quantities.

What you will be ordering

First fix consumes pipe and fittings, waste pipe, cable and back boxes, the extractor fan and its ducting. The substrate stage takes moisture-resistant plasterboard or tile backer board with their screws and tapes, levelling compound if the floor needs it, and the tanking kit (membrane, jointing tape, corners, primer) for wet areas.

The visible spend comes last: tiles with adhesive, grout and trims, the sanitaryware and brassware, a towel rail or radiator, silicone sealant, and paint for the non-tiled surfaces. Tiles are worth ordering with a margin for cuts and breakages from a single batch, so the shades match. Each calculator above returns the order quantities for its own step.

Where the regulations touch a bathroom

Every calculator, by job stage