diff --git a/docs/controller/system.md b/docs/controller/system.md
index d050b45..80ce447 100644
--- a/docs/controller/system.md
+++ b/docs/controller/system.md
@@ -88,6 +88,31 @@ The hotspot is useful for:
 - Field use without infrastructure
 - Initial setup before configuring WiFi
 
+### Captive Portal
+
+When a phone or laptop joins the hotspot, it checks for Internet access. The WROLPi answers
+that check with a welcome page instead, so the device shows a "Sign in to network" screen
+that reads:
+
+- **This is your WROLPi** and the address to open, for example `10.42.0.1`
+- A **Continue** button
+
+Tap **Continue**. The next page confirms you are connected and offers **Open WROLPi** and
+**Open the Controller**, which open in your normal browser. Tap **Done** in the corner of the
+sign-in screen to keep using the hotspot. The address is also shown under the hotspot button on
+the Controller's Control page, and `http://wrolpi/` reaches the WROLPi from any device on the
+hotspot.
+
+The welcome page does not give the device Internet access; it only tells the device the
+network is ready to use. Restarting the hotspot shows the page to every device again.
+
+**Turning it off:** uncheck **Captive portal** in **Hotspot Settings** and save. Like the
+other hotspot settings, the change applies the next time the hotspot starts, so restart the
+hotspot if it is already running. The setting is `hotspot.captive_portal` in
+`controller.yaml`.
+
+The captive portal is not available in Docker, because Docker has no hotspot.
+
 ## CPU Throttling (Raspberry Pi)
 
 Control CPU performance and power consumption on Raspberry Pi.
diff --git a/docs/index.md b/docs/index.md
index af9fd47..07b6286 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -47,6 +47,7 @@ Information about the parts of the WROLPi interface.
     * [Export](modules/inventory/export.md)
 * Calculators
     * [One-Time Pad](modules/calculators/one-time-pad.md)
+    * [Speed Test](modules/calculators/speed-test.md)
 
 ## Other File Types
 
diff --git a/docs/modules/calculators/speed-test-results.png b/docs/modules/calculators/speed-test-results.png
new file mode 100644
index 0000000..76b6458
Binary files /dev/null and b/docs/modules/calculators/speed-test-results.png differ
diff --git a/docs/modules/calculators/speed-test.md b/docs/modules/calculators/speed-test.md
new file mode 100644
index 0000000..9ea9f29
--- /dev/null
+++ b/docs/modules/calculators/speed-test.md
@@ -0,0 +1,59 @@
+# Speed Test
+
+The Speed Test measures the network between the device you are holding and your WROLPi. Use it to answer
+questions like "is the hotspot fast enough from the back porch?" or "how many people can watch videos at
+once?" without any third-party service. It works completely offline.
+
+> To open it, click **More** → **Calculators** → **Speed Test**, then click **Start test**.
+
+![The Speed Test after a run](speed-test-results.png)
+
+A run takes about 25 seconds:
+
+1. **Ping** – 20 quick round trips to the WROLPi. Reported as the median and best time, plus jitter (how much
+   the time varies between pings).
+2. **Download** – the WROLPi streams random data to your browser for 10 seconds.
+3. **Upload** – your browser sends random data to the WROLPi for 10 seconds.
+
+Nothing is saved. Reload the page and the results are gone.
+
+## Reading the results
+
+* **Mbps** is megabits per second, the unit video bitrates are quoted in. **MB/s** (megabytes per second) is
+  shown beside it; divide Mbps by 8 to get MB/s.
+* The download and upload figures are the **steady-state average**: the first second is ignored while the
+  connection ramps up. The **peak** is the fastest quarter second.
+* **Ping** under 20 ms is excellent, under 60 ms is good. Over 150 ms on a local network usually means a weak
+  WiFi signal.
+
+## Simultaneous video streams
+
+The table under the results divides your download speed by a typical bitrate for each video quality. It tells
+you roughly how many people could stream that quality at the same time over this link.
+
+| Quality | Typical bitrate |
+|---|---|
+| 480p (SD) | 2.5 Mbps |
+| 720p (HD) | 5 Mbps |
+| 1080p (Full HD) | 8 Mbps |
+| 1440p (2K) | 16 Mbps |
+| 2160p (4K) | 25 Mbps |
+
+Individual videos vary a lot; a screen recording needs far less than an action movie at the same resolution.
+
+The test measures the **network**. Videos are served from the WROLPi's drive, so a slow drive (or an SD card)
+can limit playback before the network does.
+
+## Hotspot or LAN?
+
+Under the results the page shows the address the WROLPi saw your device at. When your WROLPi's hotspot is on
+and your device has a hotspot address, it is marked **via WROLPi hotspot**. Otherwise it is marked **via LAN**.
+
+Run the test over the hotspot from different spots to find where the signal drops off, then compare with a
+wired or home-WiFi connection.
+
+## Several people at once
+
+There is no limit on how many devices can run the test at the same time. Have everyone press **Start test**
+together to see how the link shares between them; each device's results will note how many other tests were
+running during its own run.