I couldn't find any data on housing inventory over time, so I created the visual graph below for the top 24 largest metro areas.
Some interesting findings are that:
The metro areas that are outliers, fairing much better than the rest are: Miami & NYC (3.87 & 3.65 months). However, it's worth noting that Miami has trended downward significantly.
The US average for inventory days went from 3.89 months in May 2020 to 2.28 months in April 2021.
Charlotte & Sacramento remain at the bottom with 1.40 & 1.45 months.
How the Inventory Months is calculated:
Take the total listings on the market for any given day & divide that by the average monthly sales in the last 12 months.
Inventory Months = Total Listings / Avg. Monthly Sales in last 12 months.
What does the Inventory Months mean:
It measures the current supply of housing relative to demand.
In other words, if you take all the houses listed for sale currently (and not have any new listings), how many months would it take to sell all those houses based on the rate of sales in that given market.
A healthy/balanced housing market has Inventory Months between 4-6 months. Anything below 4 months indicates a Sellers' market, whereas anything above 6 months indicates a Buyers' market.