Download Time Calculator
Work out how long a file takes to transfer at a given connection speed.
What the Download Time Calculator does
Transfer time is file size divided by connection speed, once both are in the same units. The trap is that sizes are measured in bytes and speeds in bits, so a factor of eight separates them — which is why a "100 megabit" line never downloads at 100 megabytes a second.
Formula
Time = File size in bits ÷ Effective bits per secondBits = Bytes × 8Effective speed = Line speed × Efficiency1 GB = 10⁹ bytes; 1 GiB = 2³⁰ bytes
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| File size | number | Yes | Accepts more than 0. |
| Size unit | one of 9 options | Yes | — |
| Connection speed | number | Yes | Accepts more than 0. |
| Speed unit | one of 6 options | Yes | — |
| Real-world efficiency | % | Yes | Below 100% to allow for overhead and congestion. |
How to use it
- Choose Size unit and Speed unit.
- Enter File size, Connection speed and Real-world efficiency.
- Select Calculate.
Worked example
A 5 GB file over a 100 Mbps connection at full line rate.
- Size
- 5 GB
- Speed
- 100 Mbps
- Efficiency
- 100%
5 GB is 40,000,000,000 bits. At 100,000,000 bits per second that is 400 seconds — 6 minutes 40 seconds.
Reading the result
- "Time per gigabyte" is a reusable rate — since transfer time scales linearly with size at a fixed speed, this one figure lets any other file size at the same connection be estimated without recomputing.
Assumptions and limitations
- Efficiency is applied as a single flat percentage for the whole transfer — real transfers often ramp up (TCP slow-start) rather than running at full effective speed from the first byte, which a flat percentage does not model.
- "Effective speed" assumes a constant, uncongested connection for the whole duration — a long download sharing bandwidth with other traffic will see effective speed vary, not stay fixed at the entered efficiency.
Common mistakes
- Assuming a "GB" file size reported by an operating system matches this calculator's decimal GB unit — the notes explain the roughly 7% gap between GB and GiB, and OS file managers frequently report the latter while labelling it the former.
Frequently asked questions
Why is my download slower than my connection speed suggests?
Speeds are in bits and files in bytes, so divide by 8 first. Then subtract protocol overhead, server limits and congestion — 80–90% of line rate is a good day.
What is the difference between GB and GiB?
GB is 10⁹ bytes; GiB is 2³⁰ = 1,073,741,824. Storage is sold in GB while operating systems often report GiB, which is why a "1 TB" drive shows as about 931 GiB.