Calculator Methodology

Calculator index

15 calculators, one shared standard

All PlantSolve calculators operate in metric internally and convert to US customary units at output. Temperature is always converted to Celsius before any formula runs. VPD (Vapour Pressure Deficit) underpins the three highest-use calculators. Below is every calculator with its slug, followed by its full documented logic.

01Watering Frequency/calculators/watering/ 02Humidity/calculators/humidity/ 03Heat Stress Diagnosis/calculators/heat-stress-diagnosis/ 04Sunlight/calculators/sunlight/ 05Houseplant Fertilizer/calculators/fertilizer/ 06Aquarium Fertilizer Dosage/calculators/fertilizer-dosage/ 07Potting Mix/calculators/potting-mix-calculator/ 08Repotting/calculators/repotting/ 09Grow Bag Sizing/calculators/grow-bag-calculator/ 10Neem Oil Spray/calculators/neem-oil-calculator/ 11Growth Timeline/calculators/growth/ 12Sowing Calendar/calculators/sowing-calendar/ 13Mulch Calculator/calculators/mulch-calculator/ 14Compost Calculator/calculators/compost-calculator/ 15Soil pH Calculator/calculators/soil-ph-calculator/

Shared physics layer

The VPD engine

Vapour Pressure Deficit is the difference between the amount of moisture air could hold at a given temperature and how much it actually holds. It governs transpiration rate — which directly controls how fast a plant uses water. Three calculators (Watering, Humidity, Heat Stress) use VPD as their atmospheric adjustment layer.

VPD calculation (Buck equation approximation)

// Saturation vapour pressure at temperature T (kPa) satVP(T) = 0.6108 × exp( (17.27 × T) / (T + 237.3) ) // VPD (kPa) = saturation pressure × moisture deficit fraction VPD(T, RH) = satVP(T) × ( 1 − RH / 100 )

Reference baseline: VPD at 24°C, 55% RH = ~0.99 kPa. This is the "comfortable transpiration" baseline. The Watering calculator expresses all interval adjustments as a ratio of current VPD to this reference, so higher VPD (drier air) shortens watering intervals proportionally.

Calculator 01

Watering Frequency

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Computes a recommended watering interval in days using six multiplicative coefficients applied to a species base interval. VPD drives the atmospheric adjustment dynamically — this is not a lookup table.

Core formula

days = plantBase × (refVPD / currentVPD) × plantMult × potMult × sizeMult × soilMult × exposureMult // refVPD = vpd(24°C, 55%) ≈ 0.99 kPa | minimum currentVPD clamped to 0.08 // Output clamped: ferns → min 1.2 days; all others → min 1 day; max 45 days

Plant base intervals (days) + plant multipliers

Foliage tropical71.00Baseline — Pothos, Monstera, Philodendron

Succulent / cactus211.30High water storage; base interval 3× foliage

Fern40.75No drought buffer; minimum clamped to 1.2 days

Flowering50.85Active bloom increases uptake demand

Vegetable3.50.70High-demand, fast-cycling crops

Pot material multipliers

Terracotta0.72Porous walls lose ~28% more moisture than plastic

Plastic / nursery1.00Baseline — no lateral moisture loss

Glazed ceramic1.12Impermeable walls retain slightly more moisture

Self-watering1.28Reservoir system extends effective interval ~28%

Pot size multipliers

Small (<12 cm)0.65

Medium (12–22 cm)1.00

Large (>22 cm)1.55

Soil type multipliers

Sandy / fast-drain0.58

Standard mix1.00

Moisture-retaining1.45

Exposure multipliers

The exposureMult inverts intuition slightly: outdoor exposure has the lowest multiplier because outdoor plants receive natural rainfall and humidity which the calculator does not account for — so it conservatively shortens the interval.

Indoor1.35Controlled environment; no rainfall. AC/heat amplify drying.

Balcony / patio0.90Ambient humidity partially offsets pot drying

Outdoor garden0.70Ground moisture, dew, and rainfall reduce watering need

Calculator 02

Humidity

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Compares current room RH against species-appropriate target ranges, computes VPD to express whether transpiration is in a comfortable band, and assesses airflow risk for fungal issues.

Tropical foliage50 – 70%Replicates tropical forest understorey humidity

Fern60 – 80%Ferns evolved in persistently moist, shaded habitats

Succulent / cactus25 – 45%Arid origin; excess humidity encourages rot

Orchid50 – 75%Epiphytic roots need both moisture and airflow

Flowering40 – 60%Moderate humidity supports bloom without botrytis risk

VPD output thresholds: Above 2.0 kPa = "Dry-air stress likely." Below 0.4 kPa = "Air may be too stagnant or humid." 0.4–2.0 kPa = "Comfortable transpiration range." Airflow input triggers a fungal risk note when RH > 65% in still air.

Calculator 03

Heat Stress Diagnosis

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A composite scoring model — not a single formula. Points are accumulated from four independent inputs, then clamped to a 0–100 scale with three severity tiers.

Stress score accumulation

score += temperature points score += humidity points score += direct sun hours points score += symptom timing points score += VPD bonus (if VPD > 3 kPa: +8) score = clamp(round(score), 0, 100) // Special case: symptom = "all_day_wet" and score < 40 → score forced to 40

Temperature score points

≥ 42°C (107°F)+45

36–41°C (97–106°F)+35

32–35°C (90–95°F)+24

28–31°C (82–88°F)+14

Below 28°C+0

Humidity score points

≤ 25%+24

26–35%+18

36–50%+10

Above 50%+0

Direct sun hours

≥ 8 hours+20

5–7 hours+12

3–4 hours+6

< 3 hours+0

Symptom timing

All day, wet soil+26

All day, dry soil+14

Leaf scorch only+20

Midday, recovers+8

Severity tiers

0 – 39LowMonitor during temperature spikes

40 – 69ModerateFilter afternoon sun; recheck in 48 hours

70 – 100HighMove plant; morning-only watering; add shade + airflow

Calculator 04

Sunlight

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Produces a 0–100 light score that maps to a light-level label and plant suitability list. Direction sets the base; hours and distance adjust it; obstruction penalises it.

Light score formula

score = direction base score score += min(hours × 5, 35) // max +35 from sunlight hours score -= distanceMetres × 8 // −8 per metre from window score -= obstruction penalty score = clamp(score, 0, 100)

Direction base scores

Outdoor / unobstructed95

South-facing80

West-facing75

East-facing65

North-facing35

No window15

Obstruction penalties

Clear glass−0

Sheer curtain−12

Partially blocked−25

Deep / recessed−40

Score → light level bands

80 – 100Full sun / very brightBougainvillea, herbs, cactus, aloe

60 – 79Bright indirectFicus, Monstera, Peace Lily, Money Plant

35 – 59Medium indirectSnake Plant, Pothos, Lucky Bamboo

0 – 34Low lightSnake Plant, ZZ Plant, low-light Pothos

West-window note: If hours ≥ 5 and direction is west, the calculator adds: "West exposure can scorch thin leaves; use a sheer curtain in summer." This is hardcoded as a conditional advisory, not a score adjustment.

Calculator 05

Houseplant Fertilizer

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Calculates a dose in ml or grams and a feeding frequency in weeks. Three hard-block conditions override the formula entirely and output "Do not fertilize."

Hard blocks — output "Do not fertilize" when any of these are true: plant species = carnivorous (Venus Fly Trap) · growth stage = dormant · plant health = recovering · temperature > 42°C outdoors.

Dose formula (when no block applies)

raw = baseDose × (potVolumeLitres / 4) × plantDemand × stageMod × healthMult × tempMult(T) × strengthMult dose = max(0.5, round(raw, 1)) // minimum dose: 0.5 ml/g

Base doses by fertilizer type

Liquid5 ml2 weeks6 weeks

Powder / water-soluble4 g2 weeks6 weeks

Granular8 g8 weeks12 weeks

Organic liquid20 ml2 weeks4 weeks

Temperature dose multiplier — tempMult(T)

Below 12°C0.15

12 – 14°C0.30

15 – 32°C1.00

33 – 35°C0.75

36 – 38°C0.50

39 – 42°C0.35

Above 42°C0.00

Plant demand values

Pothos1.0High-N foliage feed

Monstera1.1High-N foliage feed

Peace Lily0.8Balanced NPK; bloom → High-P support

Snake Plant / Aloe0.25Very dilute balanced feed

Bougainvillea0.9Low-N active; High-P/K bloom

Hibiscus1.2Balanced with micronutrients

Rose1.15High-N early; High-P/K bloom

Venus Fly Trap0Hard-blocked — no fertilizer, ever

Calculator 06

Aquarium Fertilizer Dosage

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Based on three established dosing methodologies — Estimative Index (EI), PPS-Pro, and Low-tech. Volume is always in litres internally; US users input in gallons which are converted.

Dose formula

macro = round(tankVolume × strategyBase.macro × densityMult × co2Mult, 1) micro = round(tankVolume × strategyBase.micro × densityMult × co2Mult, 1)

Estimative Index (EI)5/20 = 0.25Alternate macro/micro days; 40–50% weekly water change

PPS-Pro (lean)7/10 = 0.70Small daily doses; keep nutrients lean

Low-tech (no CO₂)2/20 = 0.10Once weekly after water change

Plant density multipliers

Light0.75

Medium1.00

Dense1.25

CO₂ system multipliers

None0.65

Liquid / DIY0.85

Pressurised1.15

EI + no pressurised CO₂ warning: The calculator outputs: "EI dosing is high-energy. Without stable CO₂, reduce light intensity and watch for algae." This is a hardcoded conditional advisory.

Calculator 07

Potting Mix

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Outputs a 4-component substrate recipe as ratios of total volume. Drainage modifier adjusts the ratios before normalisation to ensure they always sum to 1.0.

Standard tropical55%20%10%

Aroid30%30%30%

Succulent / cactus25%50%15%

Fern50%15%5%

Flowering45%20%10%

Remaining % = worm castings / compost. Drainage modifier shifts aeration ±10% before renormalisation.

US unit note: Volume input is in quarts for US users. Component volumes are calculated directly in quarts — the code explicitly avoids converting quarts to litres to prevent double-conversion. This is documented as a critical bug fix in the source.

Calculator 08

Repotting

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Accumulative scoring model — not a single formula. Produces a verdict string at three thresholds and recommends next pot size by adding a type-specific increment to current diameter.

Score accumulation + next pot size

score += root status points score += watering speed points score += season points score += species modifier nextPotCm = currentCm + increaseCm // increaseCm: succulent = 3 cm, tree = 8 cm, all others = 5 cm

Root status points

Root rot visible+45 (forced)

Severely root-bound+60

Roots visible at drainage+30

Normal+0

Season + watering speed

Active growth season+10

Hot / summer−10

Cool / winter−15

Soil dries fast+20

Soil dries slow−15

≥ 55 (or rot present)Repot now or soon

25 – 54Monitor for 4–8 weeks

0 – 24Do not repot yet

Calculator 09

Grow Bag Sizing

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Selects the minimum-viable bag size from a lookup table of 7 standard sizes, then computes wet weight and terrace load. Load thresholds are based on common reinforced concrete balcony safe load guidelines (100–150 kg/m²).

Standard bag sizes used in lookup: 5L (20×18cm), 10L (25×25cm), 15L (30×28cm), 20L (32×35cm), 30L (40×35cm), 50L (50×40cm), 75L (60×45cm). Bags are selected as the smallest where volume ≥ plant minimum AND depth ≥ root depth minimum.

Cocopeat-heavy0.55

Standard potting mix0.70

Garden soil0.85

Comfortable load< 100 kg/m²

Approaching terrace limit100 – 149 kg/m²

High load — redistribute≥ 150 kg/m²

Calculator 10

Neem Oil Spray

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Calculates neem oil and castile soap dose from water volume and infestation level, with sensitivity downscale for delicate plants. Soap dose is always 50% of neem dose by volume.

Dose formula

tspPerL = base rate for infestation level tspPerL *= sensitivity modifier neemTsp = round(tspPerL × volumeL, 2) neemMl = round(neemTsp × 5, 1) soapMl = round(neemMl × 0.5, 1) // soap always 50% of neem volume

Base rates (tsp/L)

Preventative0.5

Mild infestation1.0

Severe infestation2.0

Plant sensitivity modifiers

Normal× 1.0

Sensitive leaves× 0.6

Succulent / cactus× 0.5

Calculator 11

Growth Timeline

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Estimates weeks to a visible growth milestone using four multiplicative modifiers. Output is a range (estimated weeks to estimated weeks × 1.35) to reflect realistic variability.

Timeline formula

weeks = clamp(baseWeeks × stageMult × lightMult × careMult × tempMult, 2, 52) // Output range: weeks to Math.round(weeks × 1.35)

Base weeks by type

Foliage8

Succulent14

Flowering10

Vegetable6

Aquatic5

Growth stage multipliers

Cutting× 1.40

Rooted cutting× 1.10

Juvenile× 0.80

Mature× 0.60

Temperature multiplier: Below 16°C or above 34°C → × 1.35 (growth slowed). 20–29°C → × 0.90 (optimal range, fastest growth). All other temperatures → × 1.00.

Calculator 12

Sowing Calendar

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Pure lookup model — no formula. Cross-references crop type (warm/cool season) against 5 USDA Hardiness Zone bands to return primary and secondary planting windows, harvest estimate, and method.

Zone 3–4 (coldest)Early June (after frost)Early May

Zone 5–6Mid-to-Late MayEarly April

Zone 7–8Mid-AprilMarch

Zone 9–10MarchOctober – November

Zone 11+ (tropical)Year-round (best Nov–Feb)December – January

10 crops covered: Tomato (75 days), Pepper (80), Okra (60), Cucumber (55), Leafy Greens (40), Cilantro (45), Radish (30), Cauliflower (75), Peas (60), Basil (50). Days-to-harvest appear in output.

Calculator 13 — Courtyard Suite

Mulch Calculator

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Computes bulk mulch volume from area × depth, with a 30% volume reduction for "around plants" coverage mode (representing realistic path exclusions).

Volume formula

volumeL = areaSqm × (depthCm / 100) × 1000 // coverage = "around_plants": volumeL × 0.70 bagsNeeded = ceil(volumeL / 50) // 50 L = standard retail bag size

Calculator 14 — Courtyard Suite

Compost Calculator

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Application depth is set by application rate, then adjusted by a soil-texture factor before volume is calculated. Acknowledges that clay soils need less compost but benefit most from deep incorporation.

Depth and volume formula

depthCm = baseDepth[application] × soilFactor[soilType] volumeL = areaSqm × (depthCm / 100) × 1000 bagsNeeded = ceil(volumeL / 40) // 40 L = standard compost bag

Application base depths

Light top-dressing0.5

Standard amendment2.0

Heavy renovation5.0

Soil texture factors

Sandy× 1.4

Loamy× 1.0

Clay× 0.7

Calculator 15 — Courtyard Suite

Soil pH Calculator

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Calculates amendment quantity to raise pH (agricultural limestone) or lower pH (elemental sulphur), adjusted for pH difference and soil texture. Based on standard lime-requirement methodology from soil science extension literature.

Amendment formula

kgPer100Sqm = baseKg × abs(targetPH − currentPH) × textureFactor totalKg = (kgPer100Sqm / 100) × areaSqm // baseKg: raise pH → 2.5 kg/100m²/pH unit | lower pH → 0.45 kg/100m²/pH unit

Soil texture factors

Sand× 0.70

Loam× 1.00

Clay× 1.60

Materials used

Raise pHPulverised (agricultural) limestone

Lower pHElemental sulphur

Hard safety limits built into output: Metric: Do not exceed 250 g/m² sulphur in a single season. US: Do not exceed 5 lb/100 sq ft sulphur. pH changes > 1.5 units trigger a "split into two applications 3 months apart" advisory.