Distance Between Cities

What it does

Pick any two of 110+ major world cities and get three things at once: the great-circle distance (km + miles), an approximate flight time, and the live time difference between them right now. Useful for trip planning, scheduling international meetings, or just satisfying a "how far is X from Y" curiosity.

Distance is computed with the Haversine formula on a spherical Earth — the same math airlines use for great-circle routing. Flight time uses a 900 km/h cruise speed plus a 30-minute buffer for taxi, takeoff, and landing. Time difference comes from each city's IANA timezone via your browser's Intl API, so daylight saving transitions are handled automatically.

How to use it

1. Pick a departure city from the first dropdown. Cities are sorted alphabetically; type the first letter to jump there.
2. Pick a destination city from the second dropdown.
3. Read the result panel: distance in kilometers and miles, estimated flight time, time difference, and current local time in each city.
4. Swap the two cities with the swap button — the math is symmetric so you'll get the same distance and flight time, but the time-difference sign flips.

How it works

Distance uses the Haversine formula:
a = sin²(Δφ/2) + cos(φ₁)·cos(φ₂)·sin²(Δλ/2)
c = 2·atan2(√a, √(1−a))
d = R·c
where φ is latitude, λ is longitude, and R is Earth's mean radius (6,371 km — WGS-84). Haversine is the standard for short-to-medium distances on a sphere; it's numerically stable for tiny angles where the older spherical law of cosines breaks down.

Flight time = distance ÷ 900 km/h + 30 minutes. The 900 km/h figure is the typical cruise speed of a 737/A320/787 over the ground; 30 minutes covers taxi, takeoff, climb, descent, and landing on a typical mid-haul flight. For 12+ hour ultra-long haul this slightly underestimates because climb/descent are a smaller share, but for most use cases it's within ±10% of published schedules.

Time difference is read live from Intl.DateTimeFormat with the IANA timezone for each city. That means DST is automatic — London → New York is 5 hours in winter, 4 in summer, and the tool gets it right without a hard-coded offset table.

When to use this tool

• Planning an international trip and want a quick distance + flight-time estimate before checking actual carriers.
• Scheduling a video call across continents and need to know the local time on the other end right now.
• Comparing two possible meeting points by total travel distance.
• Figuring out jet-lag severity (rough rule: ≥3 timezone hours of difference is when most people feel it).

When not to use it

• Booking a flight — actual flight times run 10–15% longer than great-circle estimates because of jet-stream routing, prevailing winds, and ATC restrictions. This tool gives the lower bound; check the airline.
• Driving distance — great-circle is straight-line through the Earth's surface. Roads are longer (often 1.3–1.5×). Use a routing engine for driving.
• Polar / Antarctic routes — most commercial flights avoid the poles, so the actual route can be much longer than great-circle suggests.
• Cities not in the curated list — the dataset is ~110 major cities. For obscure destinations you'll need lat/lon coordinates and a different tool.