---
title: Can we restore 500 GB in an hour? Do the maths with the Bandwidth Calculator
description: Your recovery plan says the database is back within an hour, and the backup is 500 GB in offsite storage. Over a 1 Gbps link that takes 1 hour 14 minutes, so the plan fails before anything else goes wrong. Here's how to calculate bandwidth and transfer time with the free Bandwidth Calculator, why Mbps and MB/s differ by eight, and how to measure your real throughput from the command line.
canonical: https://watchfor.io/blog/how-to-calculate-bandwidth
---

# Can we restore 500 GB in an hour? Do the maths with the Bandwidth Calculator

2026-10-04 · WatchFor Team · Guides · guides, networking, disaster-recovery

Your recovery plan says the database is back within an hour, and the backup is 500 GB in offsite storage. Over a 1 Gbps link that takes 1 hour 14 minutes, so the plan fails before anything else goes wrong. Here's how to calculate bandwidth and transfer time with the free Bandwidth Calculator, why Mbps and MB/s differ by eight, and how to measure your real throughput from the command line.

To calculate transfer time, convert the data to bits and divide by the
speed you actually get: time = size in bytes × 8 ÷ effective bits per
second. 500 GB is 4,000,000 megabits; at 1 Gbps less 10% protocol overhead
(900 Mbps), that's about 4,444 seconds, 1 hour 14 minutes. To get the
bandwidth you need, divide the other way: size × 8 ÷ time, then add the
overhead.

The free [Bandwidth Calculator](/bandwidth-calculator) does both: how long a
transfer will take on a given link, and what link a transfer needs to finish
in time, with the overhead and the bits-versus-bytes conversion built in.

## What this tool answers

*How long will this transfer take, and what speed would make it fit?* Reach
for it when planning a backup window or a restore, a data-centre or cloud
migration, a large upload to a customer, or when someone asks why a "100
Mbps" connection downloads at 11 MB/s.

## A real example

### Calculating bandwidth for a backup restore deadline

The plan on paper: if the primary database is lost, restore last night's
full backup from offsite object storage, with a recovery time objective
(RTO) of **one hour**. The backup is **500 GB**. The office and the
recovery site both have **1 Gbps** links. Nobody had ever timed a full
restore.

In the [Bandwidth Calculator](/bandwidth-calculator) we entered 500 GB,
1 Gbps and kept the default 10% protocol overhead:

![The Bandwidth Calculator in "How long will it take?" mode with 500 GB, 1 Gbps and 10% protocol overhead: transfer time at 900.0 Mbps effective is 1h 14m 4s, and the comparison table lists DSL 10 Mbps 5d 3h 27m, 4G 50 Mbps 1d 41m, Cable 100 Mbps 12h 20m 44s, Fiber 300 Mbps 4h 6m 55s, Fiber 1 Gbps 1h 14m 4s and 10 GbE 7m 24s](/blog/how-to-calculate-bandwidth-time.webp)

**1h 14m 4s at 900.0 Mbps effective.** The maths: 500 × 10⁹ bytes × 8 =
4 × 10¹² bits; 4 × 10¹² ÷ 900 × 10⁶ bits per second = 4,444 seconds. That's
only the copy, before decompressing, restoring and starting the database.

Then the other direction, on the **What speed do I need?** tab, with the
same 500 GB and a deadline of one hour:

![The Bandwidth Calculator in "What speed do I need?" mode with 500 GB, a 1-hour deadline and 10% overhead: required line rate 1.23 Gbps, equal to 154.3 MB/s sustained for 1h](/blog/how-to-calculate-bandwidth-speed.webp)

**1.23 Gbps line rate, 154.3 MB/s sustained for an hour.** A 1 Gbps link
can't do it at any efficiency.

**The answer:** the RTO was impossible with this setup, and the calculator
showed it in a minute rather than during an outage. The realistic fixes,
each checkable in the same tool: keep a recent copy closer (a local
snapshot restores at disk speed), shrink what must come back first (restore
the hot tables, stream the rest later), compress the backup (if it
compresses 3:1, 167 GB takes about 25 minutes at 900 Mbps), or get a 10 Gbps
path (7m 24s). Or change the RTO to match reality.

## How to read the results

| What you see | What it means |
| --- | --- |
| **How long will it take? / What speed do I need?** | The two modes. Size and overhead are shared; the second field is the link speed or the deadline. |
| **Data size + unit** | MB, GB, TB are decimal (10⁶, 10⁹, 10¹² bytes); MiB, GiB, TiB are binary (2²⁰, 2³⁰, 2⁴⁰). 1 GiB is about 7.4% more than 1 GB. |
| **Connection speed + unit** | Kbps, Mbps, Gbps are bits per second; MB/s is megabytes per second (8 Mbps each). |
| **Protocol overhead (%)** | The share of the line rate lost to TCP/IP, Ethernet framing, TLS and retransmits. 5–15% is typical on a healthy wired path; Wi-Fi and long distances lose more. |
| **Transfer time at X effective** | Time at line rate minus overhead; the "effective" figure is the speed it assumed. |
| **Same transfer on common connections** | The same size at 10 Mbps to 10 Gbps with the same overhead. Over a day the seconds are dropped ("5d 3h 27m"). |
| **Required line rate (incl. overhead)** | The link speed needed to finish in the deadline, plus the same in MB/s, which is what transfer tools display. |

## More ways to use it

- **Backup windows.** A nightly 2 TB backup in a 6-hour window needs a line
  rate the calculator works out before you buy the circuit.
- **Cloud migrations.** For tens of terabytes the transfer time decides
  between the network and a shipped storage appliance.
- **"Our internet is slow."** An ISP's 100 Mbps is 12.5 MB/s at most, and
  a browser showing 11 MB/s is roughly what's left after overhead.
- **Customer deliveries.** Before promising "the export will be ready in an
  hour", check how long the download takes on their connection, not yours.

## Tips and gotchas

- **Bits and bytes differ by eight.** Links are sold in bits per second,
  files are measured in bytes. Divide Mbps by 8 for MB/s: 1 Gbps is 125 MB/s
  at best.
- **GB and GiB are not the same.** Disk vendors use GB (10⁹), many
  operating systems and tools show GiB (2³⁰) and label it GB. Use the unit
  your source used, or the estimate is off by up to 10% at the terabyte
  scale.
- **One TCP stream rarely fills a fast long-distance link.** Throughput of a
  single connection is limited by its window size and round-trip time; for
  bulk copies across continents use tools that open several streams. The
  [bandwidth vs latency explainer](/blog/bandwidth-vs-latency) shows why.
- **The slowest hop wins.** Storage read speed, the remote side's upload
  limit, a VPN, a firewall doing inspection: the link is only the ceiling.
- **Measure before you plan.** The calculator assumes the link delivers its
  rate minus overhead. Test the actual path with a large download or
  `iperf3` between your own hosts.

## Check it from the command line too

`curl` reports average download speed in bytes per second, which makes a
quick real-world throughput test. Against a public 100 MiB test file:

```bash
curl -sS -o /dev/null -w 'size %{size_download} time %{time_total} speed %{speed_download}\n' \
  https://proof.ovh.net/files/100Mb.dat
```

```text
size 104857600 time 9.213476 speed 11380894
```

11,380,894 bytes per second × 8 is about **91 Mbps** from this machine to
that server. Note the file: "100Mb" in the name, but 104,857,600 bytes,
which is 100 MiB. Feed the measured speed into the calculator instead of the
advertised one and the restore estimate becomes honest.

The transfer-time arithmetic itself, in `bc`:

```bash
echo "scale=1; 500*8*1000 / (1*1000*0.9) / 60" | bc -l
```

```text
74.0
```

500 GB × 8 = 4,000,000 megabits, over 900 Mbps, is 74.0 minutes: the
calculator's 1h 14m.

## FAQ

**How do I calculate how long a file transfer will take?**
Multiply the size in bytes by 8 to get bits, then divide by the effective
speed in bits per second. 500 GB at 900 Mbps: 4 × 10¹² ÷ 9 × 10⁸ = 4,444
seconds, about 1 hour 14 minutes.

**How long does it take to transfer 1 TB over 1 Gbps?**
About 2 hours 13 minutes at the full line rate (8 × 10¹² bits ÷ 10⁹), and
roughly 2 hours 28 minutes with 10% overhead.

**What is the difference between Mbps and MB/s?**
Mbps is megabits per second, MB/s megabytes per second. One byte is 8 bits,
so 100 Mbps is at most 12.5 MB/s.

---

Canonical page: https://watchfor.io/blog/how-to-calculate-bandwidth · All articles: https://watchfor.io/blog/all.md · Site guide: https://watchfor.io/llms.txt