Calculator
Example Data Table
| Width (mm) | Height (mm) | Side clr (mm) | Top/Bottom (mm) | Frame (mm) | Rough Opening (mm) | Area (m²) |
|---|---|---|---|---|---|---|
| 1200 | 1500 | 10 | 10 / 10 | 50 | 1220 × 1520 | 1.8000 |
| 900 | 1200 | 12 | 12 / 12 | 45 | 924 × 1224 | 1.0800 |
| 1500 | 1000 | 8 | 8 / 8 | 60 | 1516 × 1016 | 1.5000 |
Formula Used
- Rough opening width:
ROw = W + 2×Cs - Rough opening height:
ROh = H + Ct + Cb - Net frame opening:
Wn = max(0, W − 2×F),Hn = max(0, H − 2×F) - Area:
A = (W×H)after unit conversion - Perimeter:
P = 2×(W + H)after unit conversion - Per-pane sizes:
Wp = Wn/(Mv+1),Hp = Hn/(Mh+1) - Glass weight estimate:
m = (An × t × ρ × L), whereAnis net glazing area (m²),tthickness (m),ρdensity (kg/m³),Lpane layers.
Symbols: W width, H height, Cs side clearance, Ct top clearance, Cb bottom clearance, F frame thickness, Mv vertical mullions, Mh horizontal mullions.
How to Use This Calculator
- Select your input unit and preferred output unit.
- Enter the overall window width and height.
- Set clearances for side, top, and bottom framing gaps.
- Provide a realistic frame thickness for net opening estimates.
- Add mullions if the design splits the glazing into sections.
- Choose pane layers and glass thickness for weight estimates.
- Click Calculate to view results above the form.
- Use CSV or PDF buttons to download the result.
Always verify manufacturer requirements and local codes before ordering.
Professional Guide
1) Why window sizing matters
Correct window sizing protects structure, finishes, and long-term performance. A window that is too tight can rack during installation, bind the sash, and crack sealant lines. A window that is too loose increases shimming time and can create uneven load transfer at the jambs and sill. This calculator connects overall frame size to practical framing numbers so you can plan openings confidently.
2) Interpreting rough openings
Rough opening width is calculated as the frame width plus two side clearances, while rough opening height adds top and bottom clearances. In residential work, side clearances often fall around 6–12 mm per side, with similar allowances at head and sill, but manufacturer instructions should govern. Use the rough opening values for framing layout, header sizing checks, and site measurement verification.
3) Understanding net frame opening
Net opening estimates the clear glazing area after subtracting frame thickness. Typical frame thickness varies by material and profile; many common systems land around 35–70 mm. This tool subtracts twice the frame thickness from width and height to approximate the clear span used for glazing and weight estimates. It is a planning estimate, not a shop drawing.
4) Mullions and pane sizing
Adding mullions divides the net opening into columns and rows. The calculator assumes equal divisions, which is suitable for early estimating and bill-of-materials discussions. If your design includes unequal lites, match the shop schedule later. Pane sizing is especially useful when selecting glass thickness, spacer systems, and handling equipment on site.
5) Glass area and weight data
Glass weight is estimated from net glazing area, glass thickness, and density. Standard soda‑lime glass density is commonly near 2500 kg/m³, which translates to about 2.5 kg per square meter per millimeter of thickness. For example, a 6 mm layer is roughly 15 kg/m²; double glazing doubles the layer weight before accounting for spacers and frames.
6) Ventilation planning
Ventilation ratio provides a quick operable-area estimate by applying a percentage to the net opening area. Sliding windows may have a lower operable fraction than casements, while awnings vary with hardware limits. Use this value for early airflow discussions and to compare options consistently across rooms.
7) Aspect ratio checks
Tall, narrow windows and very wide openings can challenge stability, transportation, and installation. The aspect ratio check uses the maximum of W/H or H/W and flags values above your chosen limit. Many teams start with limits around 2.0–3.0 for handling comfort, then refine with project constraints.
8) Quality control steps
Before ordering, confirm the measurement reference points, verify unit selections, and document clearances agreed with installers. Record mullion counts, glazing layers, and thickness assumptions. After framing, recheck plumb, level, and square; adjust shims before fastening. Clear documentation reduces rework and protects finish schedules.
FAQs
1) What is a rough opening?
A rough opening is the framed hole in the wall. It is usually slightly larger than the window frame to allow shimming, plumbing, and sealing during installation.
2) How do I choose clearances?
Start with manufacturer guidance. As a practical range, many installations use about 6–12 mm per side and similar allowances at head and sill, then adjust for site conditions.
3) Is net frame opening the same as visible glass?
No. Net opening is a simplified estimate based on frame thickness. Visible glass can be smaller due to glazing beads, gaskets, and sash overlaps. Use shop drawings for final values.
4) Why does glass density matter?
Density converts glass volume into weight. With typical glass near 2500 kg/m³, heavier units may need stronger hardware, more installers, or lifting equipment, especially for multi-layer glazing.
5) How accurate is the ventilation area?
It is an estimating aid. Actual operable area depends on window type and hardware. Use the ratio to compare options early, then confirm with product specifications.
6) What should I do if the aspect ratio warning appears?
Consider changing dimensions, adding mullions, selecting thicker frames, or revising handling plans. Very slender or wide units can be difficult to transport, install, and keep square.
7) Can I use inches and feet?
Yes. Enter values in your chosen input unit and select an output unit for reporting. Internally, lengths are converted for consistent calculations and for area and weight estimates.