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Room Areas, Paint, Tiles & Heat Loss Calculator

Floor areas, paint, tiles, skirting, volumes, daylight and heat loss from a plan of walls.

Storeys with centre-line walls {points, thicknessMm, layers}, openings {wallId, segment, kind, offsetMm, widthMm, heightMm, sillMm}, spaces {name, at, usage} and furniture. A .kerfwise.json project is converted from its kitchen plan.

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Try an example: L-shaped flat: living room, kitchen and bathroom · Rectangular room 4 × 3 m with one window and one door · Two-storey studio house with a roof and ground slab · Furniture check: a shelf placed in a door swing

More about this tool

How it works

Draw rooms as centre-line walls with thickness, doors and windows. Get the rooms the walls enclose with net floor area, gross and construction area (DIN 277 / ISO 9836 style), wall surfaces net of openings for paint, floor tiles with waste, skirting length, room volumes, an approximate average daylight factor and an approximate room heat load, plus furniture collision and clearance checks.

  • Walls are centre-line polylines with a thickness (or layers). Segments are split where walls meet (T and X junctions); at every node the offset lines of neighbouring walls are intersected, which gives mitred L corners and clean T/X joins. Rooms are the bounded faces of this planar graph, traced on the wall faces, so a stub wall inside a room is deducted correctly.
  • Areas follow the DIN 277:2021 / ISO 9836 structure: net room area NRF = sum of the room areas measured to the wall faces (NUF usable, TF technical, VF circulation by the use you assign), gross floor area BGF = area inside the outer wall faces, construction area KGF = BGF − NRF (= the wall footprints), gross volume BRI = BGF × storey height. Simplified: plaster is part of the wall thickness, door reveals and niches are not added, no separate area under low ceilings.
  • Wall surfaces = room perimeter × clear height, minus doors and windows (each side of an internal opening counts for the room on that side; optionally only openings above a size limit). Paint = (net walls + ceiling) × coats ÷ coverage. Tiles = floor area × (1 + waste) ÷ (tile + joint) area, rounded up. Skirting = perimeter minus door widths.
  • Daylight: BRE average daylight factor ADF = T·Aw·θ·M / (A·(1 − R²)) with the glazed area Aw = window area × glazed fraction, θ = 90° − obstruction angle and A = floor + ceiling + walls. Heat load: EN 12831-1 simplified — Σ A·U·ΔT per wall, window, door, floor and ceiling (internal dimensions, U + a thermal-bridge allowance for elements to the outside or ground) plus ventilation 0.34·n·V·ΔT. U-values of layered walls after EN ISO 6946.
  • Furniture (Kerfwise cabinets and appliances as bounding boxes): collisions with wall footprints, door swings and passages, other items at the same height, windows below the item top, and the clear depth in front of each floor item (bisection to 1 mm).

Approximate planning quantities, not certified: not for building permits, energy certificates, heat load sizing of a heating system or contractual area statements. Method: areas in the DIN 277 / ISO 9836 structure from wall centre lines, the BRE average daylight factor and a simplified EN 12831-1 transmission heat load (Σ A·U·ΔT). An architect or engineer would additionally measure the built rooms, apply the rules for your purpose (for example the full EN 12831-1 with ventilation and thermal bridges for heating design, or the local living-area rules for rentals) and check every number against the approved plans.

Sources

  • DIN 277:2021-08 Grundflächen und Rauminhalte im Hochbau (gross floor area BGF, net room area NRF = NUF + TF + VF, construction area KGF, gross volume BRI) (DIN Deutsches Institut für Normung)
  • ISO 9836:2017 Performance standards in building — Definition and calculation of area and space indicators (ISO)
  • Littlefair, P. J.: Site layout planning for daylight and sunlight, BRE 209 (2nd ed. 2011), appendix C: average daylight factor ADF = T·Aw·θ·M / (A·(1 − R²)) (BRE)
  • EN 12831-1:2017 Energy performance of buildings — Method for calculation of the design heat load (transmission H_T = Σ A·U·f, ventilation 0.34·V̇·ΔT; simplified here) (CEN)
  • EN ISO 6946:2017 Building components and building elements — Thermal resistance and thermal transmittance (Rsi 0.13, Rse 0.04 m²·K/W for horizontal heat flow) (CEN / ISO)

Part of Kerfwise

  • Kerfwise Cabinet & furniture CAD

    Design cabinets and furniture, get cut lists, optimised sheet layouts, costs and drawings.

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Related tools

How to use the Room Areas, Paint, Tiles & Heat Loss Calculator

Turn a plan of walls, doors and windows into room areas, wall surfaces for paint, tile and skirting quantities, room volumes, daylight and heat-loss estimates.

What it is for

Renovating a flat starts with the same questions: how big is each room, how much paint, tiles and skirting do you need, and will the room be bright and warm enough? This calculator answers them from one simple model. You describe the walls as centre lines with a thickness, place doors and windows on them, and the tool finds the rooms the walls enclose.

Areas follow the structure of DIN 277 and ISO 9836. The tool also estimates the BRE average daylight factor and a simplified EN 12831 heat load per room, and checks furniture against walls, door swings and each other.

The numbers are planning figures. They are not certified for a building permit, an energy certificate, a rental agreement or the sizing of a heating system.

Worked example

The second example is a 4 × 3 m room with 300 mm walls, a 1.5 × 1.25 m window and a 0.9 × 2.0 m door, 2.5 m clear height. The tool returns:

  • Net room area (NRF) 12.00 m², gross floor area (BGF) 16.56 m², construction area (KGF) 4.56 m², gross volume (BRI) 45.54 m³.
  • Walls: 35.00 m² gross, 3.675 m² of openings, 31.325 m² net. With the ceiling and two coats at 8 m² per litre that is 10.83 l of paint.
  • Tiles: 73 tiles of 600 × 300 mm with 3 mm joints and 10 % waste, 10 boxes. Skirting: 13.10 m, 13.76 m with 5 % waste, because the door width is left out.
  • Daylight: window-to-floor ratio 0.156 and an average daylight factor of 1.91 %.
  • Heat load: 662 W through walls, window, door, floor and roof plus 163 W ventilation, 826 W in total, or 68.8 W/m² at −12 °C outside.

The first example, an L-shaped flat with a kitchen and a bathroom, gives 33.70 m² net and 45.83 m² gross. The bathroom window is small and partly obstructed, so its daylight factor is only 0.66 %. The fourth example adds a shelf next to the kitchen door and reports a conflict: the shelf is in the swing of door d-kit.

How to read the result

  • Building totals sum every storey. NRF plus KGF always equals BGF.
  • Areas table: one row per room with its use (NUF, TF or VF), net area, perimeter and volume.
  • Finishes table: gross and net wall area, ceiling, paint, tiles, boxes and skirting per room.
  • Daylight and heat load: these are approximate. An average daylight factor of about 2 % is a common target for kitchens, 1.5 % for living rooms and 1 % for bedrooms.
  • Furniture checks: status, the clear depth in front of each item and the reason.
  • Downloads: CSV, SVG and DXF plans, OBJ and the room JSON; the link opens the Kerfwise Room planner.

Tips and common mistakes

  • Draw walls on their centre lines and let them meet exactly; a wall that stops short leaves the room open, and the tool says so.
  • Give each room a label point inside it, with a name and a use. Unnamed rooms become "Room 1", "Room 2".
  • Use wall layers with thermal conductivities to get the U-value from EN ISO 6946 instead of a default.
  • Set deductMinM2 to 2.5 if your painter bills like ATV DIN 18363 and does not deduct small openings.
  • For daylight, enter the obstruction angle of buildings opposite a window; 30° cuts the daylight factor by a third.

FAQ

Why is the gross floor area bigger than the sum of the rooms?

It is measured to the outside of the walls; the difference is the construction area, the wall footprint.

Does the heat load replace a heating engineer's calculation?

No. It adds U × area × temperature difference per surface plus ventilation, with internal dimensions and a flat thermal-bridge allowance.

Can I use a Kerfwise kitchen project as input?

Yes. Paste the project JSON: its walls, windows and doors become the room model, and its cabinets and appliances are checked as furniture.

Are door reveals and plaster counted?

No: plaster is part of the wall thickness and reveals are not added, a simplification of DIN 277.

Guide last updated:

For scripts and AI agents

  • JSON API: POST https://kerfwiseroomquantities.usefulweb.site/api/v1/calculate (same calculation as this page)
  • Interface description: OpenAPI 3.1 · AI guide: llms.txt
  • MCP server (Streamable HTTP): https://kerfwiseroomquantities.usefulweb.site/mcp · tool name kerfwise_room_quantities
Example request
curl -s -X POST https://kerfwiseroomquantities.usefulweb.site/api/v1/calculate \
  -H 'content-type: application/json' \
  -d '{…see OpenAPI examples…}'
Feedback · ideas & bug reports welcome