Windows & Doors
Curtain / Blind Size Calculator
Calculates curtain or blind dimensions by window size
Updated May 13, 2026 · Live
What this tool does
Calculates curtain or blind dimensions by window size.
Calculates curtain fabric width and drop, or blind cut width, from a {window_width_in} x {window_height_in} in. window and mount type, giving {result}.
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Formula Used
How the curtain / blind size calculator works
Calculates curtain or blind dimensions by window size. The calculator takes your dimensions and supplier rates, applies a standard US construction formula, and returns a quantity with an indicative cost. Every figure is an estimate — site conditions always move the final number.
Typical US windows and doors wastage
Windows are made to order — there is no wastage, but rough-opening tolerances of ±1/4" are standard. Measure rough openings before ordering, and confirm the installer's flashing detail. Our defaults reflect common US trade allowances, and can be adjusted upwards for non-standard geometry or downwards where experience supports a lower figure.
What this tool does not do
It does not replace a professional quote, factor regional pricing, assess structural adequacy, or confirm building code compliance. Those remain the responsibility of a suitably qualified designer, engineer, or your local building official.
On-site considerations for curtain / blind size
Replacement windows must meet NFRC ratings (U-factor, SHGC) for the climate zone. Egress windows in sleeping rooms have IRC R310 minimum dimensions — measure before ordering.
Codes and compliance
Windows and doors in new work must meet IECC U-factor and SHGC limits by climate zone (typical 0.30 U-factor in CZ 5–6). Egress and tempered-glazing requirements per IRC R310 and R308. When in doubt, file a pre-application question with your local building department — early clarity is cheaper than a corrective inspection.
Before you order
Order Energy Star–certified units with NFRC labels. Spec low-E Argon-filled glass for the climate zone — the upgrade typically pays back in 5–8 years. Cross-checking the calculator’s output against a supplier quote helps catch differences in pricing assumptions — ask for exact product specifications (grade, finish, batch number) and confirm delivery timescales against your programme.
Adjusting the defaults
Every input in this calculator is editable. Enter your own dimensions, supplier prices, and wastage allowance — the output recalculates instantly. If the defaults feel off for your region or project type, your own numbers always override them.
Using this curtain / blind size calculator alongside other BuildMetricLab tools
This calculator works best as part of a planning workflow. Pair the quantity with our project contingency, labor-hours, and material-cost calculators to build a complete estimate before you pick up the phone to a supplier. All BuildMetricLab tools run entirely in your browser — no sign-up, no data sent anywhere, and every formula is shown on-page so you can audit the math.
Sources & methodology
This tool calculates curtain and blind dimensions from the window measurements you enter. For curtains, it multiplies window width by 1.3 to determine curtain rail width, then adds a fixed allowance to window height for a sill-length drop, and separately reports a floor-length drop and a fabric width equal to twice the rail width for a gathered look. For roller blinds, it subtracts a small clearance from window width for a recess (inside) mount and adds a fixed allowance to window height for the drop below the sill. These allowances are fixed defaults rather than adjustable ranges; always confirm final cut sizes with your curtain maker or blind supplier before ordering.
Frequently asked questions
Are curtain / blind size calculator results accurate enough to order materials?
Use them as a starting estimate only. Verifying the final quantity with your supplier or contractor before ordering is good practice — site conditions, wastage and cut-offs all affect the true figure.
What allowance does this calculator use for hems and headers?
The calculator applies fixed default allowances for the curtain header, hem, and floor-length drop, and a small clearance for inside-mount blinds. These are typical trade defaults; increase them for heavier fabrics, thicker hems, or unusual mounting depths.
Is this a substitute for a professional design consultation?
No. This tool is a planning estimator. For work that affects structure, building code compliance, gas, electrical, plumbing, or drainage to a public sewer, consult a licensed contractor or design professional.
Should I choose inside mount or outside mount for my blind?
Inside mount fits within the window recess and needs adequate frame depth; outside mount is fixed to the wall or trim above the window and can make a window look larger. Check your frame depth and the blind manufacturer's minimum mount requirements before ordering.
Spotted something off?
Calculations or display — let us know.
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