Planting Calendar by Zone
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Sun Exposure & Shade Calculator
Model the real sun on your site from latitude, date and what shades it: get solar declination, noon height, day length and peak sun hours, then verify with a stick-and-shadow survey. No map or account needed.
Sun Exposure & Shade Calculator
Chlorova · updated for 2026
Sun Exposure & Shade Calculator
Enter your latitude and a date, then hit “Calculate sun” to model exposure and check it against a stick-and-shadow survey.
This models ideal sun geometry for a flat horizon using standard solar-position formulas, then discounts it by the single site-shading factor you choose. Real light also depends on terrain, the clock time and height of specific obstructions, reflected light and seasonal tree cover, none of which a latitude and one shading setting can capture. Treat the peak-sun estimate as a guide to a site's potential and confirm it with your own week of observation or the stick-and-shadow survey before committing a season to a shady corner.
A garden's single most important site variable is how much sun a spot really gets, and eyeballing it badly is why so many crops sulk. This calculator models the sun itself in your browser from plain geometry: give it your latitude and a date, tell it what shades the site, and it computes solar declination, noon height, true day length and the hours of direct sun your bed likely sees. A simple stick-and-shadow survey lets you check those numbers against the sun overhead right now, turning guesswork into measured light.
Judging a Site in a Few Fields
- Enter your site latitude in degrees north, negative for the southern hemisphere.
- Pick the date you want to model, such as an early sowing day.
- Describe what shades the site, from open sky to deep north-side shade.
- Set the height of the main obstruction, like a fence or wall in feet.
- For a live check, stand a stick in the sun and measure its shadow.
- Enter the stick height and the shadow length you just measured.
- Press Calculate sun to model the day and its shadow.
- Read the exposure verdict and the crops that suit that light level.
The Solar Geometry Behind the Numbers
The model uses three textbook solar relationships. Declination, the sun's north-south angle for that date, is 23.45 degrees times the sine of a seasonal term, so it swings from about minus 23 in the northern winter to plus 23 in summer. Noon altitude, how high the sun climbs, is then simply 90 degrees minus the gap between your latitude and that declination.
Day length comes from the hour angle at sunrise, and peak sun is that length reduced by your site shading, so a six-hour day half-blocked by a building gives about three good hours. Shadows follow from the same noon height, since a surface throws a shadow equal to its height divided by the tangent of the sun's altitude. That is why the stick survey, angle equals arctangent of height over shadow, cross-checks the whole model against the real sky.
Worked Example: A Latitude-40 Garden on the Summer Solstice
- Set latitude to 40 degrees and choose the June 21 solstice as the date.
- Leave site shading on open sky to test an unobstructed bed.
- The model reports a solar declination near plus 23 degrees.
- Noon sun altitude works out to roughly 73 degrees above you.
- True day length comes to about 14.8 hours at that latitude.
- With open sky, peak sun equals that entire day length.
- Clearing six hours, the verdict reads full sun for the spot.
- A 6-foot fence now throws only about a 2-foot shadow at noon.
- A 3-foot stick and a matching 3-foot shadow confirm a 45-degree sun.
- The surveyed angle and the modeled height agree, so the site checks out.
Sun Hours and What They Grow
The exposure bands the verdict uses, the direct-sun hours each means, and the crops that reliably perform there.
| Exposure | Direct sun | Thrives there |
|---|---|---|
| Full sun | 6 or more hours | Tomato, pepper, corn, squash |
| Partial sun | 4 to 6 hours | Lettuce, spinach, chard, beets |
| Mostly shade | 2 to 4 hours | Arugula, mint, parsley, chives |
| Deep shade | Under 2 hours | Hostas and ornamentals only |
| Site shading | Sun fraction applied | Open 1.0 down to deep 0.2 |
Key Terms
- Declination
- The sun's north-south angle for a date, driving season length and noon height.
- Noon altitude
- The maximum height the sun reaches in a day, measured in degrees up.
- Peak sun hours
- The counted hours of direct, unblocked sunlight a spot receives in a day.
- Shadow ratio
- How many feet of shadow one foot of height throws at a given sun angle.
- Stick survey
- A field check comparing a stick's height to its shadow to read the live sun angle.
Reading Light Like a Market Gardener
- Track the worst-case season, not the best, because a spot that is full sun in June may be deep shade in the low-angle light of early spring.
- Morning and evening sun are not equal, so a bed that loses its eastern light stays damp and cooler than one holding western sun.
- Trees shade far more than their trunks suggest, since a mature canopy spreads wide and the light under it is too diffuse to drive fruiting crops.
- A south-facing wall stores and reradiates heat, so a partial-sun bed against one often behaves like full sun for heat-loving plants.
- Before fighting a shady site with tolerant greens, spend one weekend watching how light actually moves across it rather than trusting a glance.
Common Mistakes and Fixes
- Assuming a bright spot is full sun when it only gets four hours of direct beam.
- Using today's high sun to judge the whole season instead of the spring date you plant.
- Ignoring that nearby trees leaf out and change the shading completely by midsummer.
- Modeling latitude but not the real obstructions, where most garden shade actually comes from.
- Reading peak-sun hours as total daylight, which differ sharply on any shaded site.
- Planting fruiting crops into a partial-sun verdict and blaming the seed when they fail.
When to Use This and When Not To
Reach for this calculator when choosing a new bed site, diagnosing why a crop underperforms, or planning what a shady corner can actually grow through a season. It models the sun's geometry and your stated shading, so it is an estimate, not a substitute for logging real light with your own eyes over a week. For hardiness, frost and timing, pair it with the frost-date and planting-calendar tools rather than expecting sun math to cover cold.
Sun Exposure & Shade Calculator: Frequently Asked Questions
Do I need a map or GPS for this?
No, the calculator works entirely from the latitude you type and the shading you describe, running the solar geometry in your browser. There is no map lookup, so you can use it anywhere, even before you have picked the exact spot.
Why use the stick-and-shadow survey?
The model assumes a clear horizon, while your yard has real obstructions, so measuring a stick's shadow gives the sun's actual angle right then. If that surveyed angle is far below the modeled noon height, something is shading your site more than you thought.
Does the date change the answer much?
Strongly, because declination and day length shift all season, so a spot in full sun at the June solstice may drop under four hours by late autumn. Model the date you actually plant, not today's date, to judge a site fairly. A single high-summer reading badly overstates the light a spring or autumn crop will truly receive.
Is my southern hemisphere latitude negative?
Yes, enter southern latitudes as negative numbers so the solar geometry resolves the right season for your location. The same math then models your summer and winter correctly without any extra settings.
Can it account for a specific tree or wall?
Only partly, since you give an overall shading level and one obstruction height rather than exact positions on a compass. It estimates how the sun moves and how much the site loses, but a precise per-hour shadow map needs dedicated site-analysis software.
Sources & Further Reading
- Solar Position and Day-Length Geometry — NOAA Global Monitoring Laboratory. Basis for the declination, altitude and day-length math this tool runs.
- Site Selection for Home Vegetable Gardens — University of Minnesota Extension. Sun-hour thresholds separating full sun, partial sun and shade crops.
- Growing Plants in Shade — Royal Horticultural Society. Practical guidance on what tolerates limited direct light.
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated
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