Plant Tools: Free Calculators for Houseplants and Shade Gardens
Five free tools for the questions that come up between waterings: is this plant safe for the cat, how much soil to buy, why the windows are wet, how many plants fill a bed, and whether a grow light is bright enough.
Each one shows its working and names its sources, from the ASPCA’s plant database to Purdue University’s light research. No sign-up, no download; they run in the browser on a phone or a desktop.
- 5free calculators
- 109plants checked against the ASPCA
- 25houseplants with light targets
- °F / °Cimperial and metric units
Tool 1 of 5
Plant Toxicity Checker
Type any common houseplant or garden plant and see whether the ASPCA lists it as toxic to cats, to dogs, or to both, with the toxin it names and the signs it lists. Filters show every plant that is toxic to cats, toxic to dogs or safe for both.
What it showsOf the 109 plants, 57 are on the ASPCA's toxic list for cats and 46 are non-toxic to both cats and dogs. Lilies, daylilies and catnip are the only entries flagged toxic to cats but not to dogs.
Sources: ASPCA Animal Poison Control Center plant database (checked October 25, 2026); U.S. FDA on lilies.
Open the Plant Toxicity Checker →
Tool 2 of 5
Potting Soil Calculator
Works out how much potting mix a pot holds in dry quarts, liters and cubic feet, and how many bags to buy. It handles round, tapered, square and rectangular pots, several pots at once, repotting into a bigger pot and a layer of filler at the bottom.
What it showsMoving a plant up one pot size, from 6 to 8 inches, takes about 2.7 quarts of fresh mix; from 8 to 10 inches, about 4.5 quarts.
Sources: Colorado State University Extension (PlantTalk) on repotting 1 to 2 inches larger; U.S. dry-quart volume.
Open the Potting Soil Calculator →
Tool 3 of 5
Dew Point Calculator
Turns room temperature and humidity into a dew point, or the reverse, in °F or °C. Add the temperature of the window glass and it says whether that pane will fog, and the highest indoor humidity it can take while staying dry.
What it showsIn a 70°F room, window glass at 40°F starts to collect condensation once humidity passes 33%. At 50°F glass the limit is 49%; at 35°F it drops to 27%.
Sources: Magnus approximation with the Alduchov and Eskridge (1996) constants, as used by the University of Miami's humidity calculator; NDSU Extension on winter humidity.
Open the Dew Point Calculator →
Tool 4 of 5
Plant Spacing Calculator
Counts how many plants fill a rectangular or round bed at a chosen spacing, in a staggered (triangular) layout or a square grid, then gives the row spacing to mark out on planting day and the cost of the plants.
What it showsUniversity of Illinois Extension's ground-cover chart counts plants for a square grid while recommending staggered rows. Buying from the chart leaves you about 13% short.
Sources: University of Illinois Extension ground-cover planting chart; staggered rows set at 0.866 times the plant spacing.
Open the Plant Spacing Calculator →
Tool 5 of 5
DLI Calculator
Converts a grow light's PPFD, or a lux or foot-candle reading from a light meter, into a daily light integral, then compares it with the target for 25 common houseplants and works out how many hours of light would close the gap.
What it showsA spot right at a north window, about 150 foot-candles, adds up to roughly 1.3 mol/m²/day even over 12 steady hours. Purdue lists about 2 as a peace lily's minimum.
Sources: Purdue University Extension HO-238-W (DLI targets and foot-candle factors); University of Illinois Extension; NIST for lux.
Open the DLI Calculator →For press and bloggers
All five tools are free to use, link to and cite. Each calculator prints its formula and sources under the result, so a number quoted from it can be checked.
| Publisher | Low Light Leaf (lowlightleaf.com), a guide to houseplants for dim rooms and gardens in the shade, written by Ellie Marsh |
|---|---|
| Launched | October 2026 |
| Tools | Plant Toxicity Checker, Potting Soil Calculator, Dew Point Calculator, Plant Spacing Calculator, DLI Calculator |
| Price | Free, no account or download |
| Data sources | ASPCA Animal Poison Control Center, U.S. FDA, Purdue University Extension, University of Illinois Extension, Colorado State University Extension, NDSU Extension, NIST, University of Miami |
| How to cite | Low Light Leaf, [tool name], lowlightleaf.com. For example: Low Light Leaf, Dew Point Calculator. |
| Contact | Hi@lowlightleaf.com |
Screenshots. Full-size images of each tool, free to use in coverage with a link to the tool:
