Deep‑Dive: How This Parking Ratio Calculator Works
This tool estimates required parking for single‑use and mixed‑use projects using transparent, planner‑grade math.
You provide a program (e.g., gross floor area, dwelling units, seats, rooms, employees) and choose a basis and ratio
for each land‑use. The calculator converts those inputs into base stalls, applies hourly occupancy profiles
to simulate shared‑parking, then layers in realistic reductions (TDM, transit proximity, on‑street credits) and a
safety factor. Finally, it checks code minimums and maximums, calculates accessible and EV requirements, and presents
a clear, auditable result with a 24‑hour demand curve.
Formulas
1) Base stalls by use
Basei = Ratioi × Factori
The factor depends on the basis you select. For area‑based ratios, the calculator uses “per 1,000 sq ft” (Imperial) or
“per 100 m²” (Metric). For unit/seat/room/employee bases, the factor is simply the quantity entered. These base values are
not yet time‑aware or reduced; they represent a conventional, single‑use estimate.
| Basis |
Example Input |
Factor Calculation |
Illustration (Ratio = 3.0) |
| per 1,000 sq ft / per 100 m² |
10,000 sq ft (or 930 m²) |
10,000 ÷ 1,000 = 10 (or 930 ÷ 100 = 9.3) |
Base = 3.0 × 10 = 30 |
| per unit |
80 dwelling units |
Factor = 80 |
Base = 3.0 × 80 = 240 |
| per seat |
120 restaurant seats |
Factor = 120 |
Base = 3.0 × 120 = 360 |
| per employee |
45 employees |
Factor = 45 |
Base = 3.0 × 45 = 135 |
2) Shared‑parking demand by hour
Demandh = Σ Basei × Occi,h
Each use has a 24‑hour occupancy multiplier between 0 and 1 (editable in the “profiles” drawer). Summing every use at each
hour produces a realistic mixed‑use curve where peaks typically occur mid‑day or evening depending on the program.
3) Reductions, credits, and peak
Adjh = max(0, Demandh × (1 − TDM − Transit) − OnStreet)
Transportation demand management (TDM) and transit proximity reduce the summed demand; on‑street credits subtract fixed stalls.
The calculator selects the peak adjusted hour and applies your safety factor and rounding rule (ceil/nearest/floor) to produce
a recommended requirement.
4) Compliance checks and stall types
Optional code minimums/maximums may override the recommendation if they are binding. Accessible parking is computed using a
summarized ADA 2010 table (verify locally); EV‑ready and EV‑installed stalls are percentage‑based. The remaining mix is split into
standard, compact, and oversize per your percentages.
| Total Stalls |
Accessible Stalls |
Van‑Accessible (included) |
EV‑Ready @ 10% |
EV‑Installed @ 4% |
| 50 | 2 | 1 | 5 | 2 |
| 120 | 4 | 1 | 12 | 5 |
| 300 | 7 | 2 | 30 | 12 |
| 600 | 12 | 2 | 60 | 24 |
Worked Example
Consider 10,000 sq ft of office (3/1,000) plus 6,000 sq ft of retail (4/1,000). Base stalls are 30 and 24 respectively.
With typical profiles, the mid‑day hour might show office at 100% and retail at 80%, yielding 30 + (24 × 0.8) = 49.2 stalls.
Apply a 10% TDM reduction and 5 on‑street credits to get (49.2 × 0.9) − 5 ≈ 39.3. Using a 10% safety factor and “ceil” rounding:
ceil(39.3 × 1.10) = 44 stalls. If the code minimum is 40 and maximum is 60, the recommendation (44) remains unchanged;
accessible/van and EV counts are then computed from 44, and the standard/compact/oversize split is applied.
Good Practice & Notes
- Always verify local ordinances; ratios, rounding, and accessibility/EV rules vary by jurisdiction.
- Use realistic occupancy curves for restaurants, entertainment, and residential to capture evening peaks.
- Document all assumptions in the exported PDF/CSV for plan review transparency.