Sun Calculator for Gardening
Determine the precise amount of direct sunlight your garden gets to choose the perfect plants.
E.g., New York is 40.71. Find yours on a map.
Sunlight hours change dramatically by season.
Height of the fence, wall, or tree casting a shadow.
Distance from your plant to the base of the obstruction.
Select the measurement unit for height and distance.
The direction of the obstruction relative to your plants.
Estimated Direct Sunlight
8.5 Hours
This spot is ideal for Full Sun plants.
Peak Sun Angle
49.0°
Obstruction Angle
31.0°
Sunlight Window
8:45 AM – 5:15 PM
Sun Path vs. Obstruction
Chart shows sun’s path (blue) vs. the shadow cast by your obstruction (red line).
What is a Sun Calculator for Gardening?
A sun calculator for gardening is a specialized tool designed to help gardeners, landscapers, and homeowners accurately determine the amount of direct sunlight a specific area of their property receives. Unlike simply guessing, this calculator uses inputs like your location (latitude), the date, and information about nearby obstructions (like fences, trees, or buildings) to model the sun’s path across the sky. By calculating the sun’s angle throughout the day and comparing it to the angle of potential shadows, it provides a precise estimate of “sun hours.”
This information is critical for successful gardening. Plant labels with terms like “Full Sun,” “Partial Shade,” and “Full Shade” are based on the number of hours of direct sun they need. Placing a sun-loving tomato plant in a spot that only gets 3 hours of sun will lead to poor growth and few fruits. Conversely, a shade-loving Hosta will scorch in a spot with 8 hours of intense afternoon sun. This calculator removes the guesswork, ensuring you can match the right plant to the right place, a practice known as Right Plant, Right Place.
The Sun Calculator Gardening Formula and Explanation
The calculator’s logic is based on fundamental astronomy and trigonometry. It determines if your plants are in direct sun by answering one question: is the sun’s angle in the sky higher than the angle created by the top of a nearby obstruction?
- Obstruction Angle (β): This is the angle from your plant to the top of the obstruction. It’s found using trigonometry: `β = arctan(Obstruction Height / Distance to Obstruction)`.
- Sun’s Altitude Angle (α): This is the sun’s angle above the horizon. It changes throughout the day and year. The core formula is: `sin(α) = sin(Φ)sin(δ) + cos(Φ)cos(δ)cos(H)`.
The calculator loops through each hour of the day, calculates the sun’s altitude (α), and if it’s greater than the obstruction’s angle (β), that hour is counted as a “sun hour.”
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| α (Alpha) | Sun’s Altitude Angle | Degrees (°) | 0° to 90° |
| β (Beta) | Obstruction Angle | Degrees (°) | 0° to 90° |
| Φ (Phi) | Your Latitude | Degrees (°) | -90° to +90° |
| δ (Delta) | Solar Declination | Degrees (°) | -23.45° to +23.45° |
| H (Hour Angle) | Time of Day relative to Solar Noon | Degrees (°) | -180° to +180° |
Practical Examples
Example 1: Vegetable Garden by a South-Facing Fence
A gardener in Denver (Latitude: 39.7) wants to plant tomatoes along a 6-foot fence on the summer solstice (June 21st). The garden bed is 8 feet away from the fence, which is to the south.
- Inputs: Latitude=39.7, Date=June 21, Obstruction Height=6 ft, Distance=8 ft, Direction=South
- Calculation: The calculator finds the obstruction angle is ~37°. On the summer solstice at this latitude, the sun is very high in the sky for many hours.
- Results: The spot receives over 8 hours of direct sun. The calculator confirms this is a Full Sun location, perfect for a vegetable garden layout.
Example 2: Balcony Garden with an Overhang
Someone in Seattle (Latitude: 47.6) has a balcony with a 5-foot deep overhang that is 8 feet high. They want to know if they can grow partial-sun plants in mid-April. The obstruction (the overhang) is to the south.
- Inputs: Latitude=47.6, Date=April 15, Obstruction Height=8 ft, Distance=5 ft, Direction=South
- Calculation: The obstruction angle is very high, at ~58°. In April, the sun’s path is lower than in summer.
- Results: The calculator estimates about 4.5 hours of direct sun, mostly when the sun is lower in the sky in the morning and afternoon. This is a classic Partial Sun scenario, suitable for many flowers and herbs but not sun-hungry vegetables.
How to Use This sun calculator gardening
- Enter Your Latitude: This is the most crucial input for accuracy. A quick search for “latitude of [your city]” will give you this number.
- Select the Date: Choose a date to analyze. To understand the extremes, check the summer solstice (~June 21) for maximum sun and the winter solstice (~Dec 21) for minimum sun.
- Measure Obstructions: Identify any object that could cast a shadow on your garden. Enter its height and its distance from your plants.
- Choose Units: Select whether your height/distance measurements are in feet or meters.
- Set Direction: Indicate where the obstruction is relative to your plants. A fence to the south of your garden will block midday sun, while one to the west blocks evening sun.
- Interpret Results: The primary result shows the total hours of direct sun. Use this to determine your garden’s light level (Full Sun, Part Sun, etc.). The intermediate values and chart help you understand *when* during the day the sun will be shining on that spot. Check our garden sunlight guide for more tips.
Key Factors That Affect Garden Sunlight
- Latitude: The further you are from the equator, the lower the sun’s angle will be, and the more dramatic the seasonal changes in daylight hours.
- Time of Year: The sun’s path changes significantly from summer to winter. A spot that is full sun in July might be full shade in December as the sun traces a much lower arc in the sky.
- Obstruction Height & Distance: The relationship between the height of an object and its distance from your garden determines the length of the shadow it casts. A tall, close object creates more shade than a short, distant one.
- Direction of Obstruction (Aspect): A south-facing garden generally gets the most sun in the Northern Hemisphere. Obstructions to the east block morning sun, while obstructions to the west block hot afternoon sun. A north-facing wall will create shade for most of the day.
- Topography: If your garden is on a slope, the direction of the slope (its “aspect”) can significantly impact sun exposure, acting like a permanent tilt towards or away from the sun.
- Vegetation: Deciduous trees can create summer shade but allow full sun in winter, a factor to consider for a four-season garden. Evergreen trees cast dense shade year-round.
Frequently Asked Questions
1. What’s the difference between “Partial Sun” and “Partial Shade”?
They often mean 4-6 hours of sun, but “Partial Shade” implies the plant prefers protection from the intense, hot afternoon sun. Morning sun is generally gentler.
2. How accurate is this sun calculator for gardening?
It’s a very accurate model based on astronomical data. However, it can’t account for cloud cover or unique atmospheric conditions. It calculates the *potential* direct sunlight on a clear day.
3. Why is my “Full Sun” area not growing tomatoes well?
The definition is 6+ hours of *direct* sun. Your area might get bright, indirect light but if the direct rays are blocked for too long, it won’t be enough for sun-loving vegetables. Use this calculator to verify the actual direct sun hours.
4. Can I use this for my indoor plants?
Yes. You can model a window as the “obstruction”. Measure from your plant to the top and bottom of the windowpane to understand how the “obstruction angle” limits direct light. A south-facing window will provide the most light.
5. What’s the best day to use for my calculations?
Analyze at least three dates for a complete picture: Summer Solstice (max sun), Winter Solstice (min sun), and one of the Equinoxes (around March 20 or Sept 22) for an average.
6. Does the unit selection (feet vs. meters) affect the formula?
No. The calculator internally converts all measurements to a consistent unit. The angle calculation `(Height / Distance)` is a ratio, so the result is the same as long as both inputs use the same unit.
7. What does “Sunlight Window” mean?
This is the estimated time range when your garden spot is receiving direct, unobstructed sunlight. It shows you if your plants are getting morning, midday, or afternoon sun.
8. The chart shows the sun path is below the obstruction line all day. What does that mean?
This indicates the spot is in “Full Shade” on the selected day, receiving less than 3 hours of direct sun (and likely none). This is common on the north side of buildings or with large overhangs, especially in winter.