Curtain size calculator

Every dimension must be a positive finite number — zero or a negative tape reading is rejected as invalid_input so a missing measurement is not treated as a valid cut. Run it free in the browser for everyday sewing, or call the API at $0.002 per successful request when a quote sheet, classroom demo, or inventory script needs the same figure.

What a curtain-size result means and what the response contains

A curtain-size result is the cut rectangle of cloth that will cover a measured opening once you have chosen how generously the hanging panel should gather and how much extra you will fold into hems. Zero inches is not treated as a valid empty window here, because a zero tape reading is almost always a missing measurement rather than a planned opening. If you need a fabric-weight conversion, a thread-count converter, or a lining-yardage planner, use a dedicated helper; this page is the curtain-size sewing landing. It is not the rod-to-sill tape reading by itself, and it is not a shopping count of fabric bolts: it is the finished width after fullness plus both side hems, the finished length after header and bottom hem, and the product of those two sides as fabric_area. This calculator answers that with a closed form. finished_width equals window_width times fullness, the amount of cloth that will gather or pleat across the opening. hem_width_total is two times side_hem, because a left fold and a right fold are both cut from the same piece. curtain_width is those two quantities added together. hem_length_total is top_hem plus bottom_hem. curtain_length is window_height plus that pair of vertical allowances. fabric_area is curtain_width times curtain_length, so a quote sheet or a classroom check can see the rectangular area without multiplying by hand. The response echoes the six normalized inputs, returns those derived fields, and includes a method tag fullness_and_hems plus a formula string so a spreadsheet or a test can repeat the same steps without guessing. There is no hidden waste factor, no lining or interlining layer, no pair-versus-single-panel switch, and no bolt-width divider: the engine either multiplies and adds six finite positive numbers or it rejects the input with invalid_input. Rejected cases include a missing window_width, window_height, fullness, side_hem, top_hem, or bottom_hem field, a non-finite value, a magnitude above the documented ceiling, and — this is the product contract — any dimension that is not a positive number.

How window, fullness, and hem fields are read so results stay reproducible

Optional leading plus signs and scientific notation on numeric strings are allowed when they parse as finite decimals; embedded spaces, hex prefixes, and non-numeric tokens are rejected with a message that names the field so a bad cell does not become a wrong cut list. Typical fullness sits around one and a half for a light pencil gather, two for a standard gathered heading, and two and a half to three for pinch pleats or a luxury fold; typical side hems sit around two to four units each side, and typical header and bottom hems sit around six to eight units, but the engine does not lock those conventions — it only uses the numbers you give. All six fields share one length unit for the whole request: inches, centimetres, or metres, as long as you do not mix them. Computation uses ordinary floating-point multiplication and addition, then rounds each numeric field to ten decimal places so goldens stay stable across runtimes. The preferred field for the opening is window_width, but aliases width, opening_width, and window_w are accepted so a form, a query string, or a spreadsheet column can reuse the same contract without renaming cells. Height prefers window_height, with aliases height, drop, opening_height, and window_h. Fullness prefers fullness, with aliases fullness_factor, gather, and gather_factor. Side hem prefers side_hem, with aliases side_allowance, hem_side, and side_hem_allowance. The header prefers top_hem, with aliases header, header_allowance, top_allowance, and heading. The bottom fold prefers bottom_hem, with aliases hem, hem_allowance, bottom_allowance, and bottom_hem_allowance. Every one of those six values must be strictly greater than zero: that is the product contract, and a missing, zero, or negative tape reading is invalid_input. Pair the free browser run on this page with the priced API when you need that identical mathematical result inside an automated quote or inventory pipeline without re-implementing the cut rule yourself.

Where curtain-size planning fits in sewing, teaching, and software

Developers call the same contract from scripts to seed deterministic test vectors for sewing calculators, to normalize user-entered windows before a quote demo, or to satisfy a product statement that demands curtain_width, curtain_length, and fabric_area fields rather than a hand-waved estimate. On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split — free interactive exploration and a priced, identical API — matches the rest of the tier-A catalog and lets you move from a classroom demo to an automated pipeline without changing the mathematical result.