So many times, when people get motivated to “do something” about their credit cards, the first thing they do is close all the cards they haven’t used in a long time.

Sounds logical: Let’s clean out the old cobwebs in our wallet!

In reality, this is a bad idea: 15% of your credit score reflects the length of your credit history, so if you wipe out old cards, you’re erasing that history. This is one of the basic credit card rules.

Plus, you’re also lowering your “credit utilization rate,” which basically means (how much you owe) / (total credit available).

For nerdy people (aka half my readers), here’s the math of your credit utilization score — plus a little-known caveat:

“If you close an account but pay off enough debt to keep your credit utilization score the same,” says Craig Watts of FICO, “your score won’t be affected.” (Most people don’t know this.)

For example, if you carry $1,000 debt across two credit cards with $2,500 credit limits each, your credit utilization rate is 20% ($1,000 debt / $5,000 total credit available).

If you close one of the cards, suddenly your credit utilization rate jumps to 40% ($1,000 / $2,500). But if you paid off $500 in debt, your utilization rate would be 20% ($500 / $2,500) and your score would not change.

A lower credit utilization rate is preferred because lenders don’t want you regularly spending all the money you have available through credit — it’s too likely that you’ll default and not pay them anything.

NOTE: If you’re applying for a major loan — for a car, home, or education — don’t close any accounts within six months of filing the loan application. You want as much credit as possible when you apply.

However, if you know that an open account will entice you to spend, and you want to close your credit card to prevent that, you should do it.

You may take a slight hit on your credit score, but over time, it will recover— and that’s better than overspending.

Bottom line? Even if you don’t use a card, keep it open. Put a small charge on it — say, $5/month — and automate it each month. This way, you ensure your card is active and maintains your credit history.