Calculator Inputs
Example Data Table
| Scenario | Windows | Size | Coats | Frame Factor | Coverage | Waste | Estimated Paint |
|---|---|---|---|---|---|---|---|
| Greenhouse refresh | 6 | 2.5 ft × 3.5 ft | 2 | 35% | 350 sq ft/gal | 10% | ~0.58 gal |
| Shed full repaint | 4 | 3 ft × 4 ft | 3 | 45% | 325 sq ft/gal | 12% | ~1.79 gal |
| Potting station upgrade | 2 | 1.0 m × 0.8 m | 2 | 30% | 10 sq m/L | 8% | ~0.52 L |
Example values are illustrative; your materials and technique can change coverage.
Formula Used
1) Total window area: A_total = (W × H) × N
2) Painted frame area: A_frame = A_total × (Frame% ÷ 100)
3) Apply coats and waste: A_final = A_frame × Coats × (1 + Waste% ÷ 100)
4) Paint required: Paint = A_final ÷ Coverage
5) Cost: Cost = Paint × Price (Primer is calculated similarly when enabled.)
How to Use This Calculator
- Choose Imperial or Metric based on how you measure window size.
- Enter window width, height, and how many windows you will paint.
- Set a realistic frame factor to exclude unpainted glass.
- Enter coats, waste percentage, and your product’s coverage value.
- Add paint and optional primer prices to estimate project cost.
- Press Calculate to view results above the form.
- Use Download CSV or Download PDF to save your report.
Project Notes and Planning
Coverage assumptions for window frames
Paint labels often quote broad coverage, but frames behave differently than flat walls. Narrow edges, corners, and glazing bars increase brush strokes and absorption. For exterior garden structures, conservative inputs like 325–375 sq ft/gal (or 8–12 sq m/L) reduce surprise shortfalls in humid or dusty conditions.
Choosing an accurate frame factor
The calculator uses a frame factor to convert total window rectangle area into paintable frame area. Typical shed and greenhouse frames land around 25–45%. Thicker stiles, wide sashes, and decorative grids push it toward 50%. Thin aluminum profiles may be closer to 15–25%.
Coats, waste, and touch-up allowance
Two coats is common for durable color on outdoor joinery, while three coats is typical for color changes. Waste includes tray loss, brush loading, and small touch-ups after hardware reinstallation. Many DIY projects perform well with 8–15% waste; complex mullions can justify 18–25%.
Primer options for garden environments
Primer is valuable on bare wood, chalking surfaces, or repaired rot-prone edges. In greenhouses, condensation and watering can stress coatings near glazing. Priming 60–100% of the frame area improves adhesion and can reduce topcoat absorption. Use the primer percentage to model spot-priming versus full preparation.
Interpreting quantities and budget
The result “Paint needed” is a functional estimate, not a mandatory purchase size. If the output is 0.7 gallons, rounding to one gallon supports future touch-ups. Compare paint and primer costs separately to decide where quality matters most. Recording notes with the download helps repeat a consistent finish later.
FAQs
1) Does this include painting the glass?
No. The estimate targets frames, sashes, and mullions. The frame factor lets you exclude glass area so your paint quantity matches real brushable surfaces.
2) What frame factor should I start with?
Start at 35% for typical wood greenhouse frames. Use 25% for slim profiles and 45% for wide trim, thick sashes, or many glazing bars.
3) How do I handle multiple window sizes?
Run the calculator once per size group, then add the paint totals. This keeps frame factor and coats consistent while reflecting real dimensions accurately.
4) Why is waste important for small parts?
Small sections increase brush loading and reloads. Trays and brushes retain product, and edges need touch-ups. A waste allowance prevents underbuying and delays.
5) When should I include primer in the estimate?
Include primer for bare wood, stained tannin-prone boards, patched repairs, or chalky old coatings. It improves adhesion and helps topcoats level on outdoor garden structures.
6) Should I round up the paint amount?
Usually yes. Round up to the next common container size so you have enough for trim ends, missed spots, and seasonal touch-ups after weather exposure.