Calculator
Example data table
| Window height | Mount | Mechanism | Lift lines | Recommended buy length |
|---|---|---|---|---|
| 60 in | Inside | Standard | 2 | ~300 in |
| 72 in | Outside | Standard | 3 | ~500 in |
| 48 in | Inside | Continuous loop | 2 | ~420 in |
| 84 in | Outside | Continuous loop | 3 | ~720 in |
| 54 in | Inside | Top-down / bottom-up | 2 | ~520 in |
Formula used
Outside mount: Drop = WindowHeight + TopOverlap + BottomOverlap − BottomClearance
Continuous loop: PerLine = 2×Drop + 2×ControlDrop + Wrap + Routing + Knot
Top-down/bottom-up: PerLine = 4×Drop + Routing + Tassel + 2×Knot + Wrap
Recommended = Total × (1 + ExtraPercent/100)
How to use this calculator
- Measure the window height using your chosen unit.
- Pick inside or outside mount, then set overlaps.
- Set bottom clearance to avoid dragging or binding.
- Choose the mechanism that matches your hardware.
- Enter lift lines and allowances for routing and knots.
- Calculate, then download CSV or PDF for your records.
Field measurements that matter
Accurate cord estimates start with dependable height data. Measure the opening from top to sill, then decide whether the blind will sit inside the frame or overlap it. For outside mounts, add top and bottom overlap to block light gaps. For inside mounts, include bottom clearance so the hem bar does not rub plants, pots, or trim. Consistent units reduce conversion errors and improve repeatability.
Allowances for routing and hardware
Lift cords rarely travel straight. They pass through headrail guides, pulleys, and ladder drums, which consume extra length beyond the drop. The routing allowance represents that hidden path and should reflect your hardware complexity. Add finishing allowance for knots, cord locks, and trimming. If a tassel or handle is used, include its length so the pull remains comfortable at working height during maintenance.
Mechanism selection and cord paths
Standard lift systems typically use a down-and-up path per lift line, so cord length scales near twice the drop plus allowances. Continuous loop designs form a loop that includes the control drop on both sides, requiring additional loop length and a small wrap reserve. Top-down and bottom-up systems route cord through multiple moving rails, increasing travel distance and raising cord demand materially.
Using spare percentage professionally
A spare percentage converts calculated length into a purchase plan. New cord can fray during threading, and small cutting mistakes are common when retying ends. Professionals often keep five to fifteen percent extra for field variability. Higher spares are useful when you expect repeated rework, worn pulleys, or multiple practice passes. The rounded purchase suggestion helps align the recommendation with common bundle sizes.
Interpreting results for safer projects
Use the per-line value to validate your setup against an existing cord sample. If your removed cord is significantly different, adjust routing or finishing allowances to match reality. For gardens, aim for smooth operation to avoid sudden drops that can disturb nearby seedlings or equipment. Follow local cord safety guidance, keep controls tidy, and store spare cord away from moisture. Saving results as CSV or PDF improves documentation for future seasonal replacements.
FAQs
How many lift lines should I choose?
Count the visible cords or ladders at the back of the blind. Small blinds often use two lines, while wider or heavier blinds may use three or more for balanced lifting.
What if I do not know the routing allowance?
Start with the default value, then compare the estimate to the old cord you removed. Increase routing when the headrail has multiple guides, drums, or long horizontal runs.
Do overlaps affect cord length for inside mounts?
Overlaps mainly apply to outside mounts. For inside mounts, focus on clearance and true opening height. The drop is shorter, so the cord requirement usually decreases.
Is the spare percentage really necessary?
Yes. Extra cord covers trimming, knots, and mistakes during threading. A modest spare also helps when cords stretch slightly after initial use or when you need to re-tie an end.
When should I use continuous loop mode?
Use it for loop-operated blinds where the cord forms a closed loop through a clutch. Enter a realistic control drop so the loop hangs at a comfortable height.
Can I use centimeters or millimeters?
Yes. Select your unit first, then enter every measurement in that unit. The calculator converts internally and returns results in the same unit for consistent purchasing.