Flood Water Depth Calculator
| Depth of rushing water | NWS verdict |
|---|---|
| 6 inches | can knock over an adult |
| 12 inches | carries away most cars |
| 2 feet (24 inches) | carries away SUVs and trucks |
| Any depth | NEVER safe to drive or walk into flood waters |
Verdicts: NWS, Turn Around Don't Drown — never drive around the barriers blocking a flooded road; the road may have collapsed under the water. TADD® yellow warning signs comply with Federal Highway Administration standards. The numbers describe rushing water: force moves the vehicle, not depth alone. Related: thunder distance calculator, wind chill calculator, heat index calculator.
Flood deaths start with a guess: drivers look at water crossing a road and guess their vehicle can take it. The National Weather Service's numbers say the guess is wrong at almost any depth — a mere 6 inches of fast-moving water can knock over an adult walking in it, 12 inches of rushing water carries away most cars, and 2 feet takes SUVs and trucks. Enter a depth and read the verdict before anyone tests it.
Two facts make the shallow numbers lethal. First, water hides the road: what looks like a puddle may be a missing road surface, which is why the NWS says never drive around the barriers blocking a flooded road. Second, the standard applies to rushing water — the force of movement, not just the depth, does the carrying, so the same inch count is more dangerous in a flash flood than in a spreading pool.
How to use
- Enter or estimate the water depth over the road — if you cannot tell, treat it as deeper than it looks.
- Read the verdict for that depth: what the water can knock over, carry or float.
- Whatever the verdict, the NWS rule holds: turn around, don't drown — never drive around the barriers.
Frequently asked questions
How much flood water does it take to carry away a car?
Twelve inches of rushing water carries away most cars, per the National Weather Service — and just two feet will take SUVs and trucks. The water does not need to be deep enough to reach the door handle; it needs to be moving fast enough to push a floating vehicle sideways.
Can six inches of flood water really knock down an adult?
Yes — six inches of fast-moving flood water can knock over an adult, which is why walking surveys are as dangerous as driving ones. Knee-deep reads as stable to the eye, but the force against your legs scales with the speed of the current, not the depth you can see.
Why does the NWS say the road may have collapsed under the water?
Floodwater scours: the current can undermine and remove the road surface itself while the water above it still looks continuous. The missing-pavement effect is why driving around a barrier on a flooded road kills drivers in vehicles that could technically ford the depth.
What is Turn Around Don't Drown?
It is the National Weather Service's registered safety campaign, marked by the yellow When Flooded, Turn Around Don't Drown signs that comply with Federal Highway Administration standards and stand as permanent road warnings. The message is the entire safety plan: find another route instead of testing the water.
Does the depth rule change for still water?
The knock-down and carry-away numbers describe rushing water, which is the common case at flooded crossings. Still water hides its own dangers — depth, holes and contamination — and rising water becomes moving water without warning during a flash flood, so the calculator's verdict is a floor, not a permission slip.