Curtain Hem Allowance Calculator

Measure the finished drop in real conditions

Start with the final hanging position, not the raw fabric. In garden rooms, sunrooms, and sheds, floors can slope and thresholds can vary. Measure at several points, choose a target drop, then add a small leveling margin so you can trim after the curtain settles. For lasting tidy finished results.

Select a header method that matches hardware

The top allowance depends on how the curtain attaches. A rod pocket needs extra depth for the pocket and heading above it, while grommets and pleated tape typically require a taller reinforced zone. Use the preset when you want a standard build, or enter a custom value for pattern-specific construction. Reinforcement tape increases thickness; allow space for smooth easy movement.

Build the hem to control drape and durability

Hem depth affects weight, privacy, and how well the panel hangs in drafts. A single fold is lighter and faster. A double fold encloses the cut edge for cleaner wear. A weighted hem adds stability, which is helpful near greenhouse vents or doors that create airflow. As a rule, deeper hems improve drape on heavier cloth, while shallow hems suit sheers.

Account for shrinkage and moisture movement

Many fabrics shorten after washing, steaming, or humid exposure. The calculator applies shrinkage to the length that must remain after finishing. If you prewash and dry the fabric the same way it will be maintained, you can set shrinkage to zero and rely more on the leveling allowance. Natural fibers shift with humidity; recheck length after hanging.

Use a repeatable cut plan and verify before sewing

After calculating, cut all panels to the same length and label them. Press folds with consistent measurements, then hang panels temporarily to confirm the drop. If the bottom edge kisses the floor or catches on planters, trim evenly and re-press the hem before final stitching. Record your settings so future panels match the first set, and keep offcuts for testing stitches and weights.