Rain Garden Size Calculator

Rain garden sizing is determined by the volume of stormwater runoff the garden needs to absorb from connected impervious surfaces (roofs, driveways, patios). Getting the size right ensures the garden fills and drains within 24–48 hours — preventing mosquito breeding while capturing the maximum runoff.

Runoff volume formula: Volume (cubic feet) = Area × Rainfall Depth × Runoff Coefficient

Rain garden size formula: Garden Area (sq ft) = Runoff Volume (cu ft) ÷ Ponding Depth (ft)

Where:

• Drainage Area — impervious surface draining to the garden (sq ft): roof, driveway, patio sections
• Rainfall Depth — design storm depth; typically 1–1.5 inches (the storm that needs to be captured)
• Runoff Coefficient (C) — fraction of rainfall that becomes runoff: 0.95 for asphalt/roofs, 0.35 for lawn, 0.15 for established garden
• Ponding Depth — how deep water pools in the rain garden; typically 0.5–1 ft (6–12 inches)
• ÷ 12 — converts inches to feet for unit consistency

Plant spacing formula: Number of Plants = Garden Area ÷ Spacing²

Typical spacing for rain garden perennials: 1.5–2 feet apart (4 sq ft per plant).

What each variable means:

• Ponding time — a well-designed rain garden drains in 12–24 hours through infiltration into native soil; if soil has poor drainage (clay), an underdrain system is needed
• Setback rules — place rain gardens at least 10 feet from house foundations (prevents basement seepage) and 25 feet from septic systems
• Berm height — the downhill edge berm holds water in; typically 4–8 inches tall

Reference: infiltration rates by soil type:

• Sandy soil: 1+ inch/hour — excellent, minimal sizing needed
• Loamy soil: 0.5–1 inch/hour — ideal rain garden soil
• Sandy clay loam: 0.2–0.5 inch/hour — acceptable
• Clay soil: <0.1 inch/hour — requires soil amendment or underdrain

Worked example: 1,200 sq ft roof draining into a proposed rain garden. Design storm: 1 inch of rain. Runoff coefficient: 0.90 (shingle roof). Ponding depth: 8 inches (0.667 ft).

• Runoff volume = 1,200 × (1÷12) × 0.90 = 1,200 × 0.0833 × 0.90 = 89.9 cubic feet
• Garden area = 89.9 ÷ 0.667 = 134.8 sq ft (roughly 12 × 11 ft)
• Plant count at 2 ft spacing (4 sq ft/plant) = 134.8 ÷ 4 = 34 plants

Plant choices: native sedges, swamp milkweed, Joe-Pye weed, native iris, black-eyed Susan — all tolerant of periodic flooding and dry periods between storms.