What is curtain drop, and how do hem allowances work?

Finished drop and cut length

Two lengths are in play. The finished drop is what you see hanging. The cut length is longer: it includes the fabric folded into the heading at the top and into the hem at the bottom. A deep double hem takes far more fabric than a narrow one, and a weighted hem on a sheer, where a weight is sewn into the bottom edge, adds little or none.

As an example, our public Curtain Calculator, linked under this answer, allows no extra fabric for a weighted hem and 200mm for a 100mm double fold hem. Other makers use different hems and different allowances, so those figures are not universal.

Where the drop is measured from

This is the part that causes mistakes. A drop can be given from the top of the track or bracket, or from the top of the fabric to the bottom of the fabric. The two differ by the height of the track and the way the curtain hangs under it. An order has to say which one it is.

Settling and floor clearance

Fabric relaxes under its own weight after it is hung, so a curtain usually ends up slightly longer than the day it went up. Makers allow for this. Speedy Blinds makes a curtain 5mm shorter than a drop ordered from the top of the bracket for that reason. Heavy fabrics and long drops settle more, so the gap you want under the curtain is worth agreeing at the start.

Hem styles

• Double fold hem. The fabric is folded twice, so the raw edge is hidden and the hem has weight. Deeper hems hang with more weight.
• Weighted hem. A slim weight runs through the bottom edge of a sheer in place of a deep hem, for a light, clean line.
• Narrow hem. A small turned hem, used where a deep one would look heavy.

Your curtain retailer takes the final measure, chooses the reference point and writes the finished drop on the order. Ask them where your drop is measured from and what floor clearance they have allowed. Speedy Blinds does not measure or quote for homeowners.