A heavy afternoon storm can put Bangkok streets under water within an hour, and the same streets can be dry again by evening, or stay flooded for days. The difference comes down to four things that work together: how hard the rain falls, how low and flat the city is, how full the canals are, and what the Chao Phraya River is doing.

1. Rain that arrives all at once

Bangkok's rainy season runs roughly from May to October, and its storms are short and intense. A single thunderstorm can drop more rain in an hour than the drains were built to carry. When 60 to 100 millimetres fall in a couple of hours, water collects on roads faster than it can reach the drains, however well they are working.

That is why flooding often follows the storm rather than the calendar. A dry week can end with one downpour that floods a whole district, which is why a rain forecast is as useful as a flood report. The Bangkok flood map shows both.

2. A city barely above sea level

Much of Bangkok sits only a metre or two above sea level on the flat delta of the Chao Phraya. Water on flat ground has nowhere to run. It spreads out and waits for the drains and pumps to move it. Low spots such as underpasses, dips in the road and small sois fill first and empty last.

Over decades, pumping groundwater also made parts of the city sink. The pumping is now tightly controlled and the sinking has slowed, but the low ground it left behind still floods more easily than its surroundings.

3. Canals that fill up

Bangkok's streets drain into a network of canals (khlong), and pumping stations push that water out towards the river and the sea. When the canals are already full, road water has nowhere to go and flooding lasts much longer.

Canal levels are the best clue to whether a flood will clear quickly. On the map's "Canal levels" tab, a gauge above 100% means the canal is over its bank. Several overflowing gauges in one zone usually mean the streets there will stay wet for a while, even after the rain stops.

4. The river and the tide

The Chao Phraya runs through the centre of the city to the Gulf of Thailand. Late in the rainy season it carries water from the north, and high tides in the Gulf push back up the river. When a full river meets a high tide, drainage into it slows, and low riverside areas can flood even without heavy local rain.

Why some areas flood more than others

Eastern and northern districts often report the most flooded roads during heavy rain. Many of them are lower, further from the main pumping routes, or built on former rice fields and wetlands that used to hold water. On the map you can filter by zone to see how your part of the city is doing right now.

What it means for getting around

  • Timing matters. Water usually peaks during and just after a storm. Waiting an hour or two can mean a much drier trip.
  • Check canals as well as roads. Full canals mean slow drainage and floods that last.
  • Avoid underpasses and dips in heavy rain; they fill first and fastest.
  • Moving water is dangerous even when shallow. If water is flowing across the road, turn around.

Before you head out on a wet day, open the Bangkok flood map to see flooded roads, citizen reports, canal levels and the rain forecast in one place. In an emergency, call the BMA flood hotline on 1555 or the police on 191.