Cover width from the arc
Bed width is treated as a chord and hoop rise as the sagitta of a circular arc. The arc length plus both side allowances gives the seamless cover width.
R = (w² + 4h²) / (8h); arc = R × 4 atan(2h / w)Low tunnel material planner
Row Cover Size Calculator
Calculate row cover width and cut length from bed width, hoop rise and anchoring allowances, then plan roll usage and hoop spacing.
Geometry reviewed against university Extension low-tunnel examples Updated 2026-08-11
Project inputs
Measure the bed and hoops
Measure the outside span at the hoop feet. Side allowance is added to both long edges; end allowance is added to both ends.
Cover and hoop plan
79.8 inrequired seamless cover width
- Cut length per bed
- 144 in
- Roll length needed
- 144 in
- Hoops per bed
- 3
- Equalized hoop spacing
- 48 in
- Hoop material per hoop
- 79.8 in
- Roll area used
- 96%
Measured layout
Cross-section and hoop plan
The preview separates the cross-section from the bed plan. Dimensions in the result, not the drawing scale, are the cut marks to use.
Arc over the bed67.8 in
Cover strips across roll1
Total cover area79.8 sq ft
Total hoops3
Material estimate, not structural design
Wind, snow, fabric weight, fastening and hoop material affect a tunnel's strength. Confirm the installation with the cover and frame instructions.
- Clear plastic can overheat quickly; provide ventilation and follow crop guidance.
- For insect exclusion, seal edges and remove or open covers when pollination is required.
- Test one hoop and one cover section before cutting an entire roll.
How the row cover estimate works
Roll layout
The calculator fits as many full cover strips as possible across the entered roll width, then rounds the number of cut-length rows up for all beds.
Hoop count and spacing
Bed length is divided by the maximum spacing and rounded up to whole intervals. Adding one gives end hoops, and the final gaps are equalized.
Row cover and geometry references
Row cover sizing FAQ
How wide should row cover be for a 4-foot bed?
It depends on hoop rise and anchoring allowance. A 4-foot bed with a 20-inch circular rise and 6 inches for anchoring on each side needs about 80 inches of cover width.
How many hoops do I need for an 8-foot bed?
With a maximum spacing of 4 feet, the calculator returns three hoops: one at each end and one in the middle. Use closer spacing when the frame or site requires it.
Why is cover width longer than bed width plus twice the hoop height?
The fabric follows a curved path. This tool calculates the circular arc rather than approximating it with two straight slopes.
Can I join two narrow pieces of row cover?
A seam may create weak points or pest gaps. The calculator flags a narrow roll and recommends a seamless width; follow the cover manufacturer's joining method if a seam is unavoidable.
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