Dirt Bike Tire Size Calculator

1. 1Tab 1 — Size Calculator: Enter any dirt bike tire code in metric (80/100-21) or inch (3.00-21) format, or click a front/rear preset button. Click Calculate to see overall diameter, sidewall height, circumference, and revolutions per mile in both inches and mm, plus a worked formula using your exact values.
2. 2Tab 2 — Compare Two: Enter Tire A (current) and Tire B (proposed). The calculator shows a to-scale SVG comparison, OD delta in inches and %, a 4-tier fitment badge (Ideal/Acceptable/Caution/Not Recommended), and a speedometer correction table at 7 speeds from 25–85 mph.
3. 3Tab 3 — OEM Lookup: Select your brand then model from the dropdowns. Instantly see the factory front and rear tire sizes, overall diameters, rim diameters, rim widths, and revs/mile for both positions — for 24 of the most popular MX, enduro, and trail models.
4. 4Tab 4 — Find by Size: Enter a target overall diameter in inches and a rim size. The calculator scans all standard metric width/aspect combinations within ±3% of your target and returns matches ranked by closest fit — the fastest way to find an equivalent metric size for any inch target.
5. 5Tab 5 — Tire Pressure: Select your riding terrain from 8 presets (MX hard pack, enduro, sand, mud, dual-sport, trials, and more). The calculator returns minimum, ideal, and maximum psi for both front and rear, with a sourced rationale note.
6. 6Tab 6 — Rim Compatibility: Enter your rim inner width in inches (1.40, 1.60, 1.85, 2.15, 2.50, or 3.00) or click a preset. See the minimum and maximum compatible tire widths plus the optimal range — preventing dangerous under- or over-width fitment.

Dirt Bike Tire Size Formulas — Complete Reference

1. READING A METRIC DIRT BIKE TIRE CODE (e.g. 110/90-19)
  First number  → Section width in millimetres: 110 mm
  Second number → Aspect ratio (sidewall as % of width): 90%
  Third number  → Rim diameter in inches: 19 in

2. OVERALL DIAMETER (OD) FROM METRIC CODE
  Step 1 — Sidewall height (mm):
    Sidewall_mm = Width × (Aspect ÷ 100) = 110 × 0.90 = 99 mm

  Step 2 — Sidewall height (inches):
    Sidewall_in = 99 ÷ 25.4 = 3.90 in

  Step 3 — Overall diameter:
    OD = Rim_in + 2 × Sidewall_in = 19 + 2 × 3.90 = 26.80 in (680.7 mm)

  Example: 80/100-21 front
    Sidewall = 80 × 1.00 = 80 mm = 3.15 in
    OD = 21 + 2 × 3.15 = 27.30 in (693.4 mm)

3. OD FROM INCH FORMAT (e.g. 3.00-21)
  Width_mm = 3.00 × 25.4 = 76.2 mm
  Aspect assumed = 100% (standard for inch-format dirt bike tires)
  Sidewall_in = 3.00 in
  OD = 21 + 2 × 3.00 = 27.00 in

4. CIRCUMFERENCE
  C = π × OD = 3.14159 × 27.30 = 85.74 in (2,177.7 mm)

5. REVOLUTIONS PER MILE
  Rev/Mile = 63,360 ÷ C_in = 63,360 ÷ 85.74 = 739 rev/mi

6. REVOLUTIONS PER KM
  Rev/km = 39,370 ÷ C_in = 39,370 ÷ 85.74 = 459 rev/km

7. SPEEDOMETER CORRECTION (when switching tire sizes)
  True_Speed = Indicated_Speed × (New_OD ÷ Old_OD)
  Example — switching from 110/90-19 (26.80 in) to 120/80-19 (27.42 in):
  True Speed @ 60 mph indicated = 60 × (27.42 ÷ 26.80) = 61.4 mph
  (speedo reads ~2.3% low — acceptable range)

8. OD DELTA % (COMPARISON)
  Δ% = ((New_OD − Old_OD) ÷ Old_OD) × 100
  Ideal: < ±1% | Acceptable: ±1–3% | Caution: ±3–6% | Not recommended: > ±6%

9. RIM WIDTH ↔ TIRE WIDTH COMPATIBILITY
  Rim inner width 1.60 in → compatible tire widths: 70–90 mm (optimal 80–90 mm)
  Rim inner width 1.85 in → compatible tire widths: 80–100 mm (optimal 90–100 mm)
  Rim inner width 2.15 in → compatible tire widths: 90–120 mm (optimal 100–110 mm)
  Rim inner width 2.50 in → compatible tire widths: 100–130 mm (optimal 110–120 mm)
  Never mount a tire more than 10 mm narrower than rim minimum — bead may not seal.