Blackout Curtain Coverage Calculator

1. Track width = opening width + (outside mount: 2×return + overlap; inside mount: overlap).
2. Required curtain width = track width × (fullness ÷ 100).
3. Effective panel width = min(usable width, nominal width) − seam loss.
4. Panels per opening = ceil(required curtain width ÷ effective panel width).
5. Coverage % = (panels × effective panel width ÷ required curtain width) × 100.
6. Cut length per panel = finished drop + header allowance + hem allowance, then apply shrinkage.
7. Total fabric length = panels × cut length × openings, then add waste allowance.

1. Measure the opening width and height accurately.
2. Select outside mount for better light sealing.
3. Set fullness between 180% and 250% for deep folds.
4. Add overlap and returns to reduce edge glow.
5. Enter your fabric width and usable panel width.
6. Click calculate and review panels, coverage, and fabric totals.
7. Download CSV or PDF to share with your tailor.

Light control for plant prep and storage

Garden sheds often double as potting rooms, tool storage, and seed-starting spaces. Stray streetlight or security lighting can disrupt dark rests, reduce screen glare, and heat the interior. This calculator converts your opening size, mount choice, and added returns into a track width that better represents real-world light leakage points.

Choosing fullness for stronger blackout folds

Blackout performance improves when the fabric forms deeper waves. Fullness is the multiplier applied to the track width, commonly 180–250% for heavy curtains. Higher fullness increases overlap between folds, limiting pinhole transmission and edge glow. The tool shows required total curtain width and then selects a practical panel count using effective panel width after seams.

Outside mount and returns for tighter sealing

Outside mount typically delivers the cleanest darkness because the fabric covers beyond the frame. Returns pull the curtain toward the side walls, reducing light spill along the edges. Center overlap helps where two panels meet. The calculator treats these as additions to track width, so your results reflect the coverage you need, not just the glass size.

Estimating fabric and liner with allowances

Cut length is based on finished drop plus header and hem allowances, then adjusted for shrinkage. Waste allowance supports trimming, pattern matching, and installation corrections. If you include blackout liner, the tool estimates liner length with a margin so the liner sits neatly behind the face fabric, avoiding peeking edges in daylight.

Using results for purchasing and installation

Use panels per opening to plan hardware spacing and stacking area. Compare coverage percentage to confirm you meet or exceed the required width. For bright patio rooms, aim for 100% coverage or higher and consider thicker linings. Export CSV for quick ordering notes, or PDF for sharing measurements with a tailor or installer. For larger openings, confirm stack-back space so panels park clear of seedlings and workbenches in daylight.

1) What coverage percentage should I target for blackout?

Target at least 100% coverage versus required width. Higher coverage helps reduce edge glow and improves fold overlap, especially in bright outdoor-facing garden rooms.

2) Is outside mount always better for blocking light?

Usually yes. Outside mount extends fabric beyond the frame, sealing gaps at the sides and top. Inside mount can look neat but may leak light around edges.

3) How do I choose fullness for heavy blackout fabric?

Start at 200% for balanced folds and stacking. Increase toward 250% if the area is very bright or you want deeper waves. Lower than 150% can look flat and leak more light.

4) Why does the calculator ask for seam loss?

When widths are joined or finished, usable width can shrink due to seams and edge treatments. Seam loss prevents underestimating panel count and reduces surprises during installation.

5) Should I include shrinkage if the fabric is prewashed?

If the fabric and liner are fully prewashed and dried, shrinkage can be reduced. If you are unsure, keep a small shrinkage value to avoid a curtain that ends up short.

6) What waste allowance is reasonable for patterned fabric?

For solid fabrics, 5–8% is common. For patterns that need matching, plan 10–20% depending on repeat size and how many panels you are making.