Sea Level Exposure Calculator
How many years a coastal site has before sea level rise and land subsidence consume its freeboard — under whatever rate of rise you choose to plan for.
Freeboard is the site's elevation above the current highest tide.
How the sea level exposure calculator works
Freeboard is the site's elevation above the current highest tide. Sea level rise eats into it from above; subsidence eats into it from below. The years to exposure are the freeboard divided by the combined rate.
The rate is the whole argument. At 4 mm a year a metre of freeboard lasts 250 years; at 10 mm it lasts 100. This page takes the rate as an input rather than choosing one, because that choice is a policy decision, not arithmetic.
Formula: years = freeboard / (rise rate + subsidence rate)
Worked examples
| Inputs | Years until the tide reaches the site | Note |
|---|---|---|
| 1.2 m at 7.5 mm combined | 160 | 160 years to the tide, 93 to a surge |
| A faster rise | 88.9 | time halves |
| A low site | 80 | surge exposure within the asset life |
FAQFrequently asked questions
What rate of sea level rise should I use?
The global mean is currently about 4.5 mm a year and accelerating; planning benchmarks of 0.8 to 1.1 m by 2100 imply 8 to 12 mm. It is a policy choice and this page leaves it to you.
What is freeboard?
The height of the site above the highest astronomical tide. It is the buffer that rise and subsidence consume.
Why include subsidence?
Because in many deltas and reclaimed areas the land is sinking faster than the sea is rising. Jakarta subsides up to 25 cm a year in places.
What is a storm surge allowance?
The extra height a storm pushes the sea to above the tide. Half a metre is modest; cyclone coasts plan for two or more.
Is the tide reaching the site the right threshold?
It is the last one. Drainage fails, groundwater rises and insurance withdraws well before the tide arrives, which is why the surge figure is the one to plan against.
Where these figures come from
- UN-Habitat — Urban Indicators Guidelines — density, green space and public space benchmarks
- World Health Organization — Urban green spaces — the 9 m² per person minimum
- ITE — Parking Generation Manual — parking demand rates by land use
- Australian Bureau of Statistics — Regional population — Australia's national statistics agency
Last checked: September 2026. Default per-capita rates are mid-range figures from national statistical agencies and utility reports; every one is an input you should replace with the local figure.