Idioms based on numbers. Select an idiom for more details. Also try Number Idioms Quiz.
If you give a ballpark figure or a ballpark estimate, you give a number which you think is fairly close to the actual one.
If someone has a one-track mind, they spend most of their time thinking about one subject.
If you do something against all odds, or against all the odds, you do it even though there were many problems and it didn't seem possible to do.
If you have to go back to square one, you have to stop and start again, usually because something isn't working as well as expected.
If you're behind the eight ball, you're in a difficult or dangerous position.
If you are dressed to the nines, or dressed up to the nines, you are wearing very smart clothes for a special occasion.
If you have forty winks, you have a short sleep, or a nap.
If you're having second thoughts about something, you're having doubts about a decision you've made.
You can say "I owe you one!" when someone has done something for you and you'd be happy to return the favour one day.
If you're in two minds about something, you can't decide what to do, or you can't decide which option is the best.
If you kill two birds with one stone, you achieve two things with the one action.
If you do something once and for all, you do it in a way that's final and it means you'll never have to do it again.
If something happens once in a blue moon, it happens very rarely.
If you say someone is "one in a million", you mean they're an exceptionally good person.
If something is par for the course, it's what you'd expect it to be.
If you put all your eggs in the one basket, you put all your efforts or resources into one person, one thing or one plan, and if things don't work out, you lose everything.
You can say "the year dot", or "the year one", when you're talking about a very, very long time ago.
If you zero in on something, you focus your attention on it.
If something is given zero tolerance, it won't be accepted even once.
A zero-sum game is a situation in which any gain by one side or person is at the expense of a loss to another side or person involved in the situation.