Money Zakat "زكاة المال" is obligatory individually. It's not per family. So each person is held accountable on what he/she owns individually not on the total of what he has and what his wife has summed together.
The calculation method is as follows, the total value of monetary and precious holdings, like: precious metals, precious stones, etc..., for an individual is above the Zakat limit "النصاب" then that individual has to pay the Zakat on that money if he/she had it for 1 lunar calendar year. There is a dispute between Scholars on how to the 1 lunar calendar year is calculated in case the value owned went below the Zakat limit through the year and came back above before the end of the year.
The Zakat limit is 85 grams of pure gold (24K). To convert that to money, you could check the gold price in your country to know how much that is. And you have to do that for all the precious metals/stones owned as well to convert it and sum it.
The amount of Zakat is 2.5% of that money.
One last think to note and that's only applicable for women, if a woman owns any gold in the form of jewelry that she use frequently. She doesn't have to pay Zakat on that jewelry at all and it's not included in the calculation.