Plants Calculator — Water Needs & Photosynthesis Estimate
Enter pot diameter, leaf area, light intensity and temperature to estimate how much water your plant needs each day and how much oxygen it produces through photosynthesis.
Location
cm
cm²
Light intensity
°C
Estimated evapotranspiration from the pot surface
- 1
Pot surface area (cm²)
π × (20 ÷ 2)² = 314.16 - 2
Evapotranspiration rate (mm/day)
1.5 + 0.2 = 1.7Base ET for location plus temperature adjustment above 18 °C. - 3
Daily water need (mL)
314.16 × 1.7 × 0.1 = 53
How does this calculator work?
Daily water ≈ pot area (cm²) × ET rate (1–8 mm/day based on location and temperature) × 0.1 mL. Photosynthesis O₂ ≈ Michaelis–Menten rate × leaf area. For a 20 cm pot at 22°C indoors with medium light and 200 cm² leaves: roughly 47 mL water/day and about 0.10 mL O₂/hour.
Formula
How this is calculated
Water need is estimated from the pot's surface area (a circle of the given diameter) multiplied by an evapotranspiration (ET) rate derived from location and temperature. Outdoor ET typically runs 4–12 mm/day; indoor, where humidity is higher and air movement lower, is usually 1–4 mm/day. The formula multiplies area (cm²) × ET (mm/day) × 0.1 to get millilitres per day (since 1 cm² × 1 mm = 0.1 mL). This is a simplified proxy — actual ET also depends on humidity, wind, soil type, and plant species, all of which the model cannot capture without additional data.
Photosynthesis output is approximated using a Michaelis–Menten light-response curve, which is a standard model in plant physiology. The maximum photosynthetic rate for a typical C3 plant (most houseplants and food crops) is about 5 µmol O₂ per square metre per second at saturating light. The rate at a given PAR (photosynthetically active radiation) level is Vmax × PAR ÷ (Km + PAR), where Km ≈ 200 µmol/m²/s is the half-saturation constant. Converting µmol to millilitres using the molar volume of an ideal gas (22.4 L/mol at STP) and multiplying by leaf area gives mL O₂ per hour.
These are estimates for orientation only, not precision horticulture. Actual water needs depend on drainage, soil mix, pot material (terracotta loses water faster than plastic), and individual species; actual photosynthesis varies with CO₂ concentration, leaf age, and plant health. Use the results as a starting guide and observe your plant's real response.
Frequently asked questions
Most houseplants do best when the top 2–3 cm of soil is allowed to dry between waterings. The calculator suggests a frequency based on estimated evapotranspiration, but always check the soil with your finger before watering — overwatering is the most common killer of houseplants.
A small potted plant with about 200 cm² of leaf area in moderate indirect light might produce 0.05–0.3 mL of O₂ per hour — a tiny amount compared to human breathing (about 200 mL/min at rest). Plants primarily regulate air humidity rather than meaningfully raising oxygen levels indoors.
It is a simple curve that captures the observation that photosynthesis rate rises steeply at low light and levels off (saturates) at high light. Rate = Vmax × PAR ÷ (Km + PAR), where Vmax is the maximum rate and Km is the PAR level at which the rate is half-maximal.
Also known as
TG we-Calculate Editorial Team. (2026). Plants Calculator — Water Needs & Photosynthesis Estimate [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/plants-calculator
TG we-Calculate Editorial Team. "Plants Calculator — Water Needs & Photosynthesis Estimate." TG we-Calculate. 2026. https://we-calculate.com/calculator/plants-calculator.
TG we-Calculate Editorial Team, "Plants Calculator — Water Needs & Photosynthesis Estimate," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/plants-calculator
@misc{wecalculate_plants_calculator, title = {Plants Calculator — Water Needs & Photosynthesis Estimate}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/plants-calculator}}, year = {2026}, note = {TG we-Calculate} }
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