Most of it would change nothing
What If All the Ice Melted?
The floating ice is not the problem. It is already in the sea.
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Step 1 of 5
Floating ice raises nothing when it melts.
Sea ice is not the question. It has already made its space in the ocean. Ice floats because it is lighter than water for its size — so a floating iceberg is already pushing aside exactly as much water as it will become. When it melts it fills the hole it was already making.
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Step 2 of 5
So the whole question is the ice on land.
Glaciers, ice caps and ice sheets are sitting on rock, holding water that is not in the ocean yet. That water has somewhere new to go.
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Step 3 of 5
Almost all of it is in one place.
Two places hold nearly all of it. Antarctica holds roughly nine tenths of the world's ice. Greenland holds most of what is left. Every mountain glacier on Earth put together is a rounding error next to them.
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Step 4 of 5
The answer is about sixty-five metres.
That is roughly a twenty-storey building. It would not be an even flood: land is still rising in places the last ice age pressed down. But on that scale most coastal cities are underwater, and Florida is gone.
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Step 5 of 5
It would take a very long time — and it has happened before.
Ice sheets take thousands of years to melt, not decades. And the planet has been ice-free before: for most of the age of the dinosaurs there were no polar ice caps at all, and the sea was far higher.
Sea level has swung by more than 100 metres between ice ages and warm periods, without any help from us.
The short version
Melting floating ice changes the sea by nothing; the sixty-five metres is all sitting on Antarctica and Greenland.
Try it yourself
Fill a glass with ice and water right to the brim. When the ice melts it will not overflow — not one drop.