THE NUMBERS, EXPLAINED
How to use this calculator.
Identical panels split the window equally. Enter usable width after side hems and seams. Each panel uses whole vertical fabric drops, joined side by side. Plain fabric only: pattern matching, shrinkage and lining are excluded.
Start with your inputs
Enter window width (cm), finished drop (cm), fullness ratio, usable width per fabric drop after side hems and seams (cm), combined top and bottom allowance (cm), finished curtain panels. Keep amounts in the units shown on each field. The example is a starting point, not a recommended value.
| Input | Example value |
|---|---|
| Window width (cm) | 200 |
| Finished drop (cm) | 220 |
| Fullness ratio | 2 |
| Usable width per fabric drop after side hems and seams (cm) | 130 |
| Combined top and bottom allowance (cm) | 30 |
| Finished curtain panels | 2 |
A worked example
With the example inputs above, fabric drops per panel is 2. Change the inputs to explore your own scenario.
| Result | Example answer |
|---|---|
| Fabric drops per panel | 2 |
| Total fabric drops | 4 |
| Cut length per drop (cm) | 250 |
| Fabric length to buy (m) | 10 |
| Vertical joins | 2 |
How this is calculated
Fabric drops per panel
Round up (window width × fullness ÷ panel count ÷ usable drop width).
Total fabric drops
Multiply drops per panel by panel count.
Cut length per drop (cm)
Add the top-and-bottom allowance to the finished drop.
Fabric length to buy (m)
Multiply total drops by cut length; divide centimetres by 100.
Vertical joins
Each panel has one fewer vertical join than its number of drops.
Symbols and units
- a
- Window width (cm)
- b
- Finished drop (cm)
- c
- Fullness ratio
- d
- Usable width per fabric drop after side hems and seams (cm)
- e
- Combined top and bottom allowance (cm)
- f
- Finished curtain panels
Use the units shown. A list keeps the order in which you enter its values. The symbol ^ means “raised to the power of”.
Assumptions and limits
Identical panels split the window equally. Enter usable width after side hems and seams. Each panel uses whole vertical fabric drops, joined side by side. Plain fabric only: pattern matching, shrinkage and lining are excluded.
Calculations use the values you enter. Blank or invalid inputs show an error instead of silently substituting zero. Results are displayed to useful precision; calculations retain the underlying numeric precision.
Questions about this tool
What does the result tell me?
Fabric drops per panel; Total fabric drops; Cut length per drop (cm); Fabric length to buy (m); Vertical joins. The result applies to the method and inputs described above. Check the units and assumptions before using it in a decision.
Why might another calculator differ?
Differences in units, rounding, timing or model definitions can produce different answers. Compare the input definitions and the method, rather than only the final number. For this tool: Identical panels split the window equally. Enter usable width after side hems and seams. Each panel uses whole vertical fabric drops, joined side by side. Plain fabric only: pattern matching, shrinkage and lining are excluded.
Are my inputs stored or sent to AI?
Calculations on this page happen in your browser. The separate AI question finder sends its submitted conversation to OpenAI; these calculator fields are not sent. Favourites and recent tool names are stored locally. A shared link contains the inputs you choose to include.
Further reading
Sailrite: curtain fullness and fabric planning ↗
Background reading; this does not imply endorsement or independent review of SumFizz.
Review status
Software example and browser checks are recorded in our quality report. No external expert review is claimed. Created with coding assistance; calculations run as ordinary browser code.
Read the quality report →