When we buy stuff, we want to know we’re getting the most for our money.

So why don’t we demand the same thing with our charitable donations? If we can buy twice as much good for the same money — or three times as much, or a thousand — why don’t we?

“There’s a very emotional aspect to giving,” said Michael Thatcher, the president and chief executive of Charity Navigator, the website most used to evaluate charities. “We are touched by a situation and somehow want to impact it.”

We give with our hearts. But we should also give with our heads. Every problem we care about has many, many different organizations aiming to solve it. Which should get our money?

Say you care about clean water. A $50 donation to a charity called Give Clean Water can provide someone with a year of clean water. But the same donation to Evidence Action can buy 100 people a year of clean water. Same cause — but with a different charity, your gift does 100 times as much. It’s as if one of the charities wasted 99 percent of its money.