Download Time Calculator

How long will your file take to download?

⚡ The #1 download confusion: Mbps ≠ MB/s

Your ISP advertises speed in Megabits per second (Mbps). Your files are measured in Megabytes (MB). Since 1 Byte = 8 bits:

100 Mbps ÷ 8 = 12.5 MB/s actual transfer rate

This is why a "100 Mbps" connection downloads a 1 GB file in ~80 seconds — not 10 seconds. Enter your speed in Mbps below and this calculator does the conversion automatically.

Common file sizes:
Speed presets:
Protocol overhead (10–15%)
TCP/IP headers and retransmissions reduce effective throughput by ~15%. Toggle to see realistic vs. theoretical time.

Speed Comparison

Speed Connection type Download time (advertised) With 15% overhead Relative speed
🔢 Unit Converter & Cheat Sheet
Quick conversions:
1 MB = 8 Mb (Megabits)  ·  1 GB = 1,000 MB  ·  1 TB = 1,000 GB
100 Mbps ÷ 8 = 12.5 MB/s  ·  1 Gbps = 125 MB/s  ·  10 Mbps = 1.25 MB/s
Note: These use decimal SI units (1 GB = 10⁹ bytes), which ISPs and most operating systems use. Windows historically used 1 GiB = 2³⁰ bytes, causing apparent discrepancies in reported file sizes.

How Download Time Is Calculated

The core formula is simple: divide the total number of bits in the file by the connection speed in bits per second.

Time (seconds) = File size in bits ÷ Speed in bits per second

Step-by-step example: 4 GB file at 50 Mbps

  • File size in bytes: 4 × 1,000,000,000 = 4,000,000,000 bytes
  • File size in bits: 4,000,000,000 × 8 = 32,000,000,000 bits
  • Speed in bps: 50 × 1,000,000 = 50,000,000 bps
  • Time: 32,000,000,000 ÷ 50,000,000 = 640 seconds = 10 minutes 40 seconds
  • With 15% overhead: 640 × (1 / 0.85) ≈ 753 seconds = 12 minutes 33 seconds

Why real-world speeds are lower than advertised

Every packet sent over TCP/IP carries header data, acknowledgment packets, and may be retransmitted if lost. Network congestion, router processing, and Wi-Fi interference add further delays. Collectively, these reduce your effective throughput to roughly 80–90% of the advertised speed under typical conditions. The "15% overhead" toggle in this calculator applies a conservative 15% reduction to model real-world expectations.

Mbps vs. MB/s — The Biggest Confusion in Downloads

This confusion accounts for a huge number of "my internet is slow" complaints. Here is the complete picture:

  • Mbps = Megabits per second — how ISPs sell and advertise plans (because the numbers look bigger).
  • MB/s = Megabytes per second — how your download manager, operating system, and browser show transfer rate.
  • 1 Byte = 8 bits, so: Mbps ÷ 8 = MB/s

Real example: You pay for "200 Mbps" broadband. Your browser shows a download at "22 MB/s". Is this slow? No — 22 MB/s × 8 = 176 Mbps out of 200 Mbps. That is 88% efficiency, which is excellent.

The "lower-case b vs upper-case B" distinction exists throughout the networking industry and is used intentionally by ISPs to advertise larger-looking numbers. This calculator always treats your input as Mbps (bits) and converts internally to bytes.

Frequently Asked Questions

Why is my download slower than my internet speed?
Internet speeds are advertised in Megabits per second (Mbps), but files are measured in Megabytes (MB). Since 1 Byte = 8 bits, a 100 Mbps connection transfers at most 12.5 MB/s. Protocol overhead (TCP headers, retransmission, routing) further reduces this by 10–15%. So 100 Mbps in practice delivers about 10–11 MB/s of real file data.
What is the difference between Mbps and MB/s?
Mbps (Megabits per second) is how ISPs advertise speed. MB/s (Megabytes per second) is how download tools show transfer rate. Convert by dividing by 8: 100 Mbps ÷ 8 = 12.5 MB/s. The capital "B" vs lowercase "b" distinction is the key — Bytes are 8× larger than bits.
How long does it take to download 1 GB on 100 Mbps?
At exactly 100 Mbps: 1 GB × 8 ÷ 100 Mbps = 80 seconds (1 minute 20 seconds). With 15% protocol overhead, expect about 94 seconds (roughly 1 minute 34 seconds). Use the calculator above for any size/speed combination.
How long does it take to download a 4K movie?
A typical 4K movie file is 15–25 GB. At 100 Mbps without overhead: 25 GB ≈ 33 minutes. At 50 Mbps ≈ 67 minutes. At 25 Mbps ≈ 2 hours 13 minutes. Enter your specific file size and speed above for a precise answer.
What is protocol overhead and why does it matter?
Network protocols add header data, error-checking, and acknowledgment packets beyond the raw file payload. TCP/IP overhead is typically 10–15% of bandwidth. This means a 100 Mbps connection delivers ~85–90 Mbps of actual data. For precise planning, always use the "with overhead" figure.
How do I calculate download time manually?
Formula: Time (seconds) = File size in bits ÷ Speed in bps. Convert: 1 GB = 8 × 10⁹ bits; 50 Mbps = 50 × 10⁶ bps. Example: 4 GB at 50 Mbps = 32 × 10⁹ ÷ 50 × 10⁶ = 640 seconds = 10 min 40 sec.