Inputs
in
Measure at top, middle, bottom; enter the smallest.
in
Measure left, center, right; enter the smallest.
Applies to output sizing.
Inside Mount Clearances
Defaults: 3/8" width, 1/2" height
in
Total deduction (both sides combined).
in
Top/bottom combined (if needed for sill clearance).
Outside Mount Overlaps
Defaults: 2" left/right, 3" top, 2" bottom
in
in
in
in
Results
Recommended Blind Width
— in
— mm
Recommended Blind Height
— in
— mm
Suggested Slat Size
—
Based on opening height for balanced look and stacking depth.
Example Data Table
Editable — add your current inputs via “Add to Example Table”.
| # | Mount | Units | Open W | Open H | Rec. W | Rec. H | Slat |
|---|---|---|---|---|---|---|---|
| 1 | inside | in | 36 | 60 | 35.625 | 59.5 | 2" |
| 2 | outside | in | 30 | 48 | 34 | 53 | 2" |
| 3 | inside | mm | 1000 | 1200 | 990 | 1187 | 25 mm |
Formulas Used
- Unit Conversion: mm = in × 25.4, in = mm ÷ 25.4
- Inside Mount: Blind Width = Opening Width − Total Width Clearance; Blind Height = Opening Height − Total Height Clearance
- Outside Mount: Blind Width = Opening Width + Left Overlap + Right Overlap; Blind Height = Opening Height + Top Overlap + Bottom Overlap
- Rounding (nearest 1/8"): round(x × 8) ÷ 8; nearest 1/16": round(x × 16) ÷ 16; mm: round(x) after conversion.
- Slat Size Suggestion (rule of thumb): If height < 36" ⇒ 1" slats; 36–60" ⇒ 2" slats; > 60" ⇒ 2.5–3" slats (or 50–63 mm).
Notes: Actual manufacturer deductions may vary. For precise ordering, use the smallest opening dimension and consult product specifications for headrail and bracket allowances.
How to Use This Calculator
- Select Units and choose Mount Type.
- Measure the window opening at three points horizontally and vertically; enter the smallest width and height.
- For inside mount, keep the default clearances or enter the manufacturer’s recommended total deductions.
- For outside mount, set overlaps for left, right, top, and bottom to ensure full coverage and light block.
- Choose an output Rounding method, then click Calculate.
- Review Suggested Slat Size and any warnings; adjust as needed.
- Use Add to Example Table to capture the current scenario, then export with CSV or PDF.
FAQs
It varies by product. Common total width deductions are around 1/4" to 3/8" (6–10 mm). Height deductions may be minimal. Always verify the exact deduction for your specific blind.
A typical starting point is 2" (50 mm) each side and bottom, and 3" (75 mm) at top to clear the headrail and reduce light gaps. Increase overlaps for blackout performance.
Follow your supplier’s ordering increments. Many accept 1/8" steps; some metric-only catalogs prefer whole millimeters. This tool lets you choose either approach.
Shallower windows and shorter heights look balanced with 1" (25 mm) slats. Taller openings benefit from 2" (50 mm) or 2.5–3" (63–76 mm) for fewer lines and better view-through.
If inside-mount clearances are very small, blinds may rub due to out-of-square frames or seasonal movement. Increase the deduction slightly or confirm squareness at multiple points.