When you take out a mortgage to buy your primary home in the Netherlands, the mortgage interest is deductible (if you meet certain conditions). However, mortgage interest deduction in the Netherlands doesn't work in quite the same way as it does in the U.S.. This article describes how it does work, and the accompanying calculator allows you to compute how much of a deduction (if any!) you can expect for your specific case.

Understanding the *Eigenwoningforfait* or Notional Rental Value
When you own your home in the Netherlands, then from an income tax perspective your home is assumed to generate taxable income. This taxable income is the notional rental value (*eigenwoningforfait* in Dutch). The notional rental value (NRV) of your home is calculated as follows:

1. Look up the WOZ value of your home.
2. On the belastingdienst site look up the NRV % that corresponds to the WOZ value of your home.
3. Multiply the WOZ value by the NRV % to get the NRV.

The NRV is added to your box 1 income. In other words, the NRV increases your tax burden.
The NRV is taxed at your marginal tax rate.

Deducting your mortgage interest
While the NRV increases your taxable income, your mortgage interest reduces your tax burden. As you can in the U.S., you can deduct your mortgage interest from your taxable income.

How much of your mortgage interest can you deduct?
While the NRV is taxed at your marginal tax rate, you may not be able to deduct your mortgage interest at your marginal tax rate, further reducing the effect of your morgage interest deduction.
In 2023 the rate for mortgage income deduction has been capped at 36.93%. This means that if you make more than €73,031 your marginal tax rate is 49.5%, but you cannot deduct your mortgage interest at that rate. Note that 36.93% happens to be the box 1 income tax rate for income below €73,031.

So, if you make more than the income threshold, your NRV is taxed at your marginal rate of 49.5%, but your mortgage interest can only be deducted at 36.93%.

Will my mortgage interest deduction result in a tax refund?
Your tax refund isn't simply the size of your mortgage interest deduction because of the tax owed on your NRV.
If your mortgage interest deduction is more than the tax you owe on your NRV, you may (depending on the other factors in your tax return) get a refund from the belastingdienst.
On the other hand, if the tax you owe on your NRV is greater than or equal to your mortgage interest deduction, you will either get no refund, or even end up owing the belastingdienst when you file your taxes!

As the WOZ value of your house increases, your refund will smaller.
As you pay off your mortgage and pay less interest over time, your refund will get smaller.

Disclaimers
I am not a tax professional, and this is my hobby project - not to be misconstrued as tax advice.