Patio Lighting Calculator

Related Calculators

Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Lumen: Light made by a bulb or fixture. In the calculator, Brightness per light (lumens) is the lumen rating for one light.

Foot-candle: Light landing on one square foot. One foot-candle means 1 lumen on 1 square foot.

Light that reaches the patio (factor): A number from 0.10 to 1.00 that estimates how much bulb light actually lands on the patio after direction, shade, dirt, and fixture losses.

Lux: The metric version of light landing on a surface. The calculator also shows Target light level in lux for comparison with metric lighting guides.

Kilowatt-hour (kWh): A unit of electrical energy. Energy use per month uses total watts, hours per night, and average days per month.

Light color (K): Color temperature in kelvin. Lower numbers such as 2700 K look warmer and more yellow; higher numbers look cooler and more blue-white.

Mistake: Using the whole backyard for Patio length (ft) and Patio width (ft).
Fix: Measure only the patio area you want lit, or use dimensions that give about the same lit area.

Mistake: Entering watts in Brightness per light (lumens).
Fix: Find the lumen number on the package for Brightness per light (lumens), and put watts in Power per light (watts).

Mistake: Choosing How bright should it feel? as Use my own level, then leaving Your target light level (foot-candles) blank.
Fix: Enter a positive foot-candle target, or switch back to Soft hangout, Dinner or games, or Bright tasks.

Mistake: Setting Light that reaches the patio (factor) above 1 or below 0.10.
Fix: Use a factor from 0.10 to 1.00. Lower values fit blocked, sideways, or dirty fixtures.

Mistake: Treating Approximate edge spacing as an exact layout for every light style.
Fix: Use Approximate edge spacing as a quick check for edge lights. For string lights or wall lights, adjust placement for doors, outlets, posts, and glare.

Mistake: Leaving Electricity price ($ per kWh) filled with a guess and reading Estimated cost per month as exact.
Fix: Use the rate from your bill, or leave Electricity price ($ per kWh) blank and use Energy use per month instead.

• The patio is treated as a rectangle. For curves, L-shapes, or several seating zones, enter an equivalent area or calculate each zone separately.
• Brightness is an average over the patio. Real patios can have bright spots, shadows, glare, and darker corners.
• Light that reaches the patio (factor) is a planning estimate, not a measured fixture efficiency. Aim, shade, height, dirt, walls, and nearby plants can change it.
• Approximate edge spacing assumes lights are spread around the patio perimeter. It is less exact for overhead strings, wall sconces, pendants, or lights grouped in one area.
• Energy use per month uses 30.4375 days per month and the Use time (hours per night) you enter. Seasonal use, dimmers, timers, and motion controls can change real use.
• Estimated cost per month is shown only when Electricity price ($ per kWh) is filled in. Taxes, tiered rates, fees, and time-of-use pricing are not included.
• The Lighting check is a practical note, not a code review or professional lighting design. Local rules, neighbors, safety needs, and wildlife-sensitive areas may need stricter choices.

Core calculation

The calculator first finds the lit patio area, then multiplies that area by the target light level. A foot-candle is lumens per square foot, so this area method estimates the bulb output needed before rounding to whole lights.

area_sq_ft = patio_length_ft * patio_width_ft

target_fc = 2, 5, 10, or custom_target_fc

total_lumens_needed = (target_fc * area_sq_ft) / light_reach_factor

lights_to_buy = ceil(total_lumens_needed / lumens_per_light)

Rounding uses the next whole light because you cannot buy part of a light.

Brightness and spacing check

After rounding up the light count, the calculator checks the average installed brightness and a simple edge spacing guide.

installed_brightness_fc = (lights_to_buy * lumens_per_light * light_reach_factor) / area_sq_ft

total_lumens_installed = lights_to_buy * lumens_per_light

edge_spacing_ft = (2 * (patio_length_ft + patio_width_ft)) / lights_to_buy

If installed brightness is much higher than the target, lower-lumen bulbs, a dimmer, or fewer higher-aimed zones may feel better. The responsible lighting note also checks for very cool light or a setup that may use more light than needed; directing home outdoor light only where it is needed is part of responsible outdoor lighting guidance [1].

Power, cost, and lux

Power and energy are based on the rounded-up light count, entered watts per light, and the entered nightly use time.

total_power_w = lights_to_buy * watts_per_light

energy_per_month_kwh = total_power_w * hours_per_night * 30.4375 / 1000

cost_per_month = energy_per_month_kwh * electric_rate_per_kwh

target_lux = target_fc * 10.7639

The monthly cost is skipped when Electricity price ($ per kWh) is blank. The lux result is a metric comparison for the target brightness level.

Mini-example

For a 12 ft by 10 ft patio, the area is 120 square feet. With Soft hangout at 2 foot-candles and a light reach factor of 0.80, the target bulb output is (2 * 120) / 0.80 = 300 lumens. If each light is 100 lumens, the result is ceil(300 / 100) = 3 lights. Installed brightness is (3 * 100 * 0.80) / 120 = 2 foot-candles, and edge spacing is 2 * (12 + 10) / 3 = 14.67 ft between lights.