Window Size Calculator, South Africa
Check every room's windows for light, ventilation and the space outside, then the energy rules per storey, against SANS 10400-O and SANS 10400-XA, with a printable room schedule.
Quick answer: A room's window area, frames included, must be at least 10% of its floor area for light, or 5% in a category 1 building (up to 80 m²) of class F2, H3 or H4. Openable area must be at least 5%. A 12 m² bedroom in a category 1 house needs 0.6 m² of each (SANS 10400-O:2011).
Window Size Calculator
SANS 10400-O:2011 light and ventilation · SANS 10400-XA:2021 fenestration
Quick start: typical rooms
Energy: per storey (optional)
This tool uses SANS 10400-O (2011 edition) and SANS 10400-XA (2021 edition) to generate its estimates. Standards can change without notice, so please confirm the current edition with your building engineer before you build or order.
| Room | Floor (m²) | Window, Have / Need (m²) | Opens, Have / Need (m²) | Result |
|---|
| Storey | Windows (m²) | Nett Floor (m²) | Ratio | Required |
|---|
How to Use This Calculator
- Pick a quick start or choose a modeTap a typical room to fill the form, check your own windows, or switch to the smallest window a room needs.
- Choose the building class and sizeTake the class from your approved plans, and say whether the building is category 1, which means up to 80 m² of floor area with no basement.
- Add each roomGive its storey, its type and its floor area inside the walls. The room type sets the length of the space outside the window.
- Add the windows in each roomChoose the style and a standard size or your own, then the quantity and the side of the building it faces.
- Set how the window opensFor a hinged window give the number of opening sashes, their size and the opening angle; the calculator turns these into an estimated openable area.
- Add the space outside, if you know itOpen the zone of space section, say what limits the space, then give the height and the available length.
- Add the energy details, if you want themGive each storey's nett floor area, any roof lights, and whether the north-side shading meets the standard.
- Click Check My WindowsResults show a pass or a shortfall for light and ventilation in every room, the zone of space result, the energy requirement per storey and a room schedule you can print.
Light and ventilation are two separate tests
South African building regulations ask two different things of a habitable room's windows, and a window that passes one can fail the other. SANS 10400-O, the lighting and ventilation part of the National Building Regulations, sets the light test in clause 4.2.1.1.3 and the ventilation test in clause 4.3.1.1.3. A window with mostly fixed glass lets in plenty of light but may open too little to ventilate the room.
The light test counts the whole window area, frames and glazing bars included. The ventilation test counts only the area that opens, together with any door or other opening in an external wall. Because the tests differ, the calculator shows both side by side for every room and says which one fails.
As an example, take a 4 m × 3 m bedroom in a small house, so a floor area of 12 m². In a category 1 dwelling house, light needs 5% of 12 m², which is 0.6 m² of window including frames. Ventilation also needs 5%, which is 0.6 m² that opens. A 1,200 × 1,200 mm top-hung window has a frame area of 1.42 m², so light passes comfortably. If it has two 600 × 600 mm sashes and a stay that opens them to 45°, the openable area is 0.36 m², a shortfall; with both sashes fully open at 90°, it is 0.72 m² and passes.
Window area needed by building size
The percentages depend on whether the building is category 1. Clause 3.6 of SANS 10400-O defines it as a building of occupancy class A3, A4, F2, G1, H2, H3 or H4 with no basement, no more than 6.0 m between intersecting walls or supports, and no more than 80 m² of floor area. A house larger than 80 m² is therefore not category 1, and its rooms follow the larger percentage for light. Floor area also drives zoning, which the Floor Area Ratio Calculator covers separately.
| Building | Light, Window Area | Ventilation, Opening Area |
|---|---|---|
| Category 1, class F2, H3 or H4 | 5% of floor area | 5%, or 0.1 m² |
| Category 1, class E4 or H5 | 10%, or 0.2 m² | 5%, or 0.1 m² |
| Any other building | 10%, or 0.2 m² | 5%, or 0.2 m² |
"Or" means the larger of the two figures applies. The 0.2 m² floor for light is written into the 10% limb of the clause, so the calculator applies it there only. Where the standard names a class that no category 1 building can hold, such as E4 or H5 under category 1, the calculator follows the clause as written.
The zone of space outside the window
Natural light only counts if there is open space in front of the window, which the standard calls a zone of space (clause 4.2.1.2). Its required length is a fraction of H, and H is the height of what limits the light. With nothing in the way, H is the vertical distance from the head of the window to the top of its wall. Where an obstruction such as a boundary wall or a neighbouring building cuts the zone, H is the obstruction's height above the head of the window, and the obstruction must be at least 1 m away.
| Room served by the window | Length of Zone |
|---|---|
| Habitable room in a dwelling house, dwelling unit, residential or institutional building | 1/3 H |
| Any other habitable room | 1/5 H |
| Bathroom, shower room or room with a toilet pan or urinal | 1/10 H |
The available length is measured to the boundary line or the building line, and it is half the sum of the lengths of the two planes through the head and the sill of the window. It must be at least 0.5 m to a boundary and 1 m to a building line, and it need never be more than 8 m. For example, a bedroom window with its head 2.1 m above the floor looks onto a 4 m high wall 2 m away. H is 1.9 m, so one third of H is 0.63 m, and 2 m of available length passes. At 0.6 m away, the same wall fails the 1 m minimum.
A roof overhang or other projection above the window can reduce the light as well. Clause 4.2.1.2.8 then asks for an unrestricted vertical shaft over at least two-thirds of the zone's plan area. The calculator checks the zone length only, so ask your designer about a deep projection.
How the calculator estimates the openable area
Part O counts the area of an openable window, but the two makers' catalogues we used do not publish the free opening of their standard sizes, and a window that opens only a little lets in less air than one that swings fully out. The calculator therefore works the openable area out by simple arithmetic and labels it as an estimate. For a top-hung or side-hung window, it takes the area of the opening sashes and multiplies it by the opening angle divided by 90, so a sash that opens to 45° counts as half and one that opens to 90° counts in full.
This approach leans cautious. For a sash hinged at one edge, the real gap is wider than the angle-divided-by-90 share at every angle between 15° and 90°; at 45°, simple geometry gives about 76%, against the 50% used here. The openable area it reports is therefore a safe figure, and a narrow fail may still pass on site. A sliding window counts as half its area, because one pane slides over the other. A door or vent counts for ventilation at the clear opening area you enter. Where you choose a standard aluminium size, the calculator suggests one sash up to 600 mm wide, which matches the layouts the makers draw, and you can change the count and the sizes.
Windows and energy: SANS 10400-XA:2021
SANS 10400-XA sets the energy performance of windows for new buildings and additions of the occupancy classes it lists (A1 to A4, C1, C2, E1 to E4, F1 to F3, G1 and H1 to H5). The 2021 edition replaced the old rule that fenestration up to 15% of the floor area complies. It now compares the total window area on each storey, frames included, with that storey's nett floor area, and Table 4 gives the maximum U-value and solar heat gain coefficient (SHGC) for the ratio.
| Windows / Nett Floor Area | U-Value (W/m².K) | SHGC, Shaded | SHGC, Unshaded |
|---|---|---|---|
| Up to 20% | Any solution | ||
| Up to 25% | 5.20 | 0.66 | 0.49 |
| Up to 30% | 4.40 | 0.53 | 0.44 |
| Up to 35% | 3.80 | 0.49 | 0.40 |
| Up to 40% | 3.30 | 0.44 | 0.35 |
| Up to 45% | 3.00 | 0.40 | 0.33 |
| Up to 50% | 2.60 | 0.35 | 0.30 |
| Up to 55% | 2.40 | 0.33 | 0.28 |
| Up to 60% | 2.20 | 0.30 | 0.25 |
| Over 60% | 2.00 | 0.28 | 0.22 |
The SHGC columns apply to windows facing west, north-west, north, north-east and east. Windows facing south-west, south and south-east have no SHGC limit, but the U-value still applies. Each storey and each sector stands alone, and a good result on one storey cannot make up for another. The values must be certified for the whole window, glass and frame together, by the glazier or another competent person, so ask your glazier for the certified figures. Roof lights follow their own rule in clause 5.3.8: acceptable up to 1.5% of the floor area they serve, with limits in Table 5 up to 10% and a U-value of 2.0 and SHGC of 0.25 above that.
What this check leaves out
- Balconies, galleries, verandahs and courts: a room that opens onto one has extra rules in Part O (clauses 4.2.1.3 and 4.3.1.2) that are not modelled.
- Flue ventilation: an opening connected to a vertical ventilating flue needs only 2% of the floor area (clause 4.3.1.1.4), which is not modelled.
- Artificial lighting and ventilation: Part O allows both in some rooms instead of natural openings; this tool checks the natural route only.
- Whole-building energy routes: SANS 10400-XA also allows a certified energy consumption calculation or a reference building model, and the energy zone of your town; none of these is modelled.
- Glazing safety, fire and security: other parts of SANS 10400 and your municipality may add rules. Your building plans need municipal approval before you build.
- Budget: the Building Cost Calculator shows where windows sit in a house budget, and the Roof Trusses Calculator covers the roof above them.
Frequently Asked Questions
Three sets of rules meet at the window. SANS 10400-O sets the minimum window and opening area for light and ventilation in every habitable room. SANS 10400-XA sets the energy performance of the glazing for the building classes it covers. SANS 10400-N sets the rules for the glass itself. Your municipality's building control office checks all three when it approves your plans.
There is no fixed size; the standard sets a share of the floor area. A bedroom in a category 1 house needs window area of 5% of its floor area, so a 10 m² bedroom needs 0.5 m². In a larger house the share is 10%, so the same bedroom needs 1.0 m². Ventilation needs openings of 5% of the floor area in both cases, with a 0.1 m² or 0.2 m² floor depending on the building.
Every habitable room needs natural light and ventilation from openings in an external wall or in the roof, unless the regulations allow artificial lighting and ventilation for it. Treat bedrooms, lounges, kitchens and offices as habitable rooms; your approved plans show how each room is classed. Check the artificial route with your building control officer before you leave a window out.
One window is enough when it meets both areas: window area including frames for light, and an openable area for ventilation. A wide fixed pane with a small opening sash can pass light and fail ventilation, so a second opening sash or a larger opening angle may be what fixes it. Enter the window and its sashes above, and the calculator shows the shortfall in square metres.
Not always. A natural opening is one route; the other is artificial lighting and ventilation, which is how internal bathrooms are usually handled. Where a bathroom or toilet does rely on a window, Part O sets the same percentages for light and ventilation as for any room served by a natural opening, with a shorter zone of space for a bathroom or toilet (one tenth of H, instead of one third). Confirm the route with your building control officer.
Work out the floor area, then find the larger of the percentage figure and the minimum for your building. For light, add up the window areas including frames. For ventilation, add up the areas that open, doors and vents included. Compare each total with its minimum. Then check that each window has enough open space outside it. The calculator does these steps for every room and shows the clause beside each result.
Divide the total area of the windows on a storey, frames included, by that storey's nett floor area and multiply by 100. For example, 14 m² of windows on a 70 m² storey is a ratio of 20%. SANS 10400-XA uses this figure to look up the maximum U-value and SHGC; up to 20% any glazing solution is acceptable. The ratio is separate from the floor area ratio used in zoning.
Fenestration is every part of the building envelope that lets in light: windows, glazed doors, skylights and the frames around the glass. SANS 10400-XA measures it as the whole element, glass and frame together, because the frame changes how much heat passes through. That is why the area used for the energy check includes the frames, and why the certified values must come from the glazier.
Ask your glazier for the certified U-value and SHGC of the whole window, then compare them with the Table 4 row for the storey's window-to-floor ratio, separately for north-facing and south-facing windows. Up to 20%, any glazing solution is acceptable. Enter your rooms above to see each storey's row.