Canopy Cover Calculator
Enter a tree's crown diameter and your bed's dimensions to estimate percentage coverage, and optionally its height and the sun's angle to find the shadow length.
Canopy cover
Reading your own canopy
To measure crown diameter without a ladder, stand at the tree's trunk, walk out to the edge of the canopy's shadow at midday, and pace or measure that distance — doubled, it's a reasonable diameter estimate. Do this for a couple of directions and average them if the crown is lopsided.
Once you know your coverage percentage, pair it with the light level calculator for a lux-based reading, and use the shade plant selector to shortlist plants suited to that category.
Frequently Asked Questions
How is canopy area worked out from a diameter?
A tree's crown is treated as a circle, so its area is π times the radius squared (half the diameter, squared). It's a simplification — real canopies are rarely perfect circles — but it's a solid working estimate for deciding how much of a bed sits under cover.
What does the coverage percentage mean for plant choice?
Under 20% covered, a bed is mostly dappled or lightly shaded and can often still take part-sun perennials. From 20–50% it's genuinely partial shade; 50–80% is full shade suited to classic shade perennials; above 80% it's deep shade where only the most shade-tolerant species — many ferns, some groundcovers — will thrive.
How does the shadow length calculation work?
It's basic right-triangle trigonometry: shadow length equals the tree's height divided by the tangent of the sun's angle above the horizon. A low sun (small angle) casts a long shadow; a high midday sun (angle near 90°) casts a short one. This is separate from canopy cover, which stays constant through the day — shadow length is about where that shade falls at a particular moment.
Does canopy cover change through the year?
Yes, dramatically under deciduous trees — a bed can be nearly full sun in early spring before leaf-out and deep shade by midsummer under full canopy. Evergreen canopy cover stays much more constant year-round. Measure your canopy diameter at the time of year that matters most for your planting plan.
Educational tool only. Canopy shape and shadow paths in the real world are irregular — treat these figures as planning estimates, not survey-grade measurements.