Modal verbs for probability

A modal verb is a word that expresses mood or attitude toward the action verb.

Obligation – must, have to, ought to, be supposed to, should
Volition – want to, would like to, intend to
Ability – can, could, be able to
Desire – want, want to, would like to, like to, love to, would love/hate to
Probability – may be, might be, could be, can be, will be.

Using modal verbs to express probability is sometimes a source of confusion for English learners.

The modal verbs below are the indicative case. They indicate a fact or actuality.


100% 90% 80% 70% 60% 50%

shall will want to must can may

If the Almighty says it shall rain, then it shall rain with 100% certainty.

If the weatherman says it will rain, it will rain with about 90% certainty. (He’s only human.)

If there are dark, stormy clouds in the sky and the wind is blowing hard, we might say “It wants to rain,” but the probability is only around 80% that it will indeed rain.

If it hasn’t rained in a while, and the humidity is high, you can say “it must rain,” but the probability that it will (do as it must) is only around 70%.

If the weatherman has said there is a 60% chance of rain, then be prepared because it can rain even if you hope it won’t.

If the weather report calls for only a 50% chance, then you have more reason to hope, but it may still rain in spite of your optimism.


The modal verbs below are the subjunctive case. They express what is imagined or wished for. 

Each indicative modal verb has a subjunctive partner which cuts the probability in half.

50% 45% 40% 35% 30% 25%
should would would like to ought to could might
While a declaration using “shall” shows certain and definite intentions, using “should” throws the intention into complete doubt. 

If a smoker says “I should quit smoking,” there is a 50% chance that he/she won’t do it, so it sounds doubtful. However, if your boss says “I should give you a raise,” it might sound hopeful to you. Beware though that there is still only a 50% chance that you will get that raise, so don’t count on it too heavily.

If you ask a friend to help you do a task, and they reply “I would..,” then you know you will soon hear the word “but..,” and the chances that your friend will actually help you are only 45% – less than half, but still a possibility. Instead of “but” the next word could be “if” which means that if certain conditions are met, your friend would indeed come through for you.
If, however, your friend says “I would like to..,” you can be sure that the next word is “but” and that the likelihood of getting help from this friend is only 40%.

Returning to the smoker who says “I ought to quit smoking,” you can be fairly certain that he/she has little intention of quitting. Even if they must (70%), using the words “ought to” drops the probability to a mere 35% leaving major doubt about the smokers intention to quit.

Returning to the weather forecast, if the weatherman predicts a 30% chance of rain, we might not be too concerned that it will rain on our picnic, but we should be prepared because it could still rain that day.

If the forecast is only for 25%, few of us will worry, but there is still a (slim) chance that it might rain on our picnic nevertheless.

Modal Verb Probability Chart

100% 90% 80% 70% 60% 50%
shall will want to must can may
50% 45% 40% 35% 30% 25% 
should would would like to ought to could might

The chart above shows the relationships of probability among the modal verbs. The subjunctive partner of each indicative modal verb cuts its probability in half. When using modal verbs to express probability the relationships are somewhat different from the basic meanings. The basic meanings of “must” and “should” are quite close together when they both express obligation. If your parents tell you that you “should” behave, they are politely saying that you “must” behave. According to the probability chart, however, those two are far apart from each other, so they can’t be used as synonyms of each other.If you suspect that a situation exists, the strongest probability that you can assign that suspicion is 70%. That is, the situation must exist. (- as far as you know – Otherwise, you could say that it does exist.) Saying “the situation should exist,” at 50%, drops the probability a full 20% below “must exist,” so it isn’t interchangeable in this sense.

Something must be the matter with Molly today. She may be angry about something. She could be upset about her exams, but she should have passed them easily. She can‘t be upset about her upcoming birthday, because she should be getting that car she wanted. She must be happy about that.
She could be upset about her computer. It must be broken again. She just had it repaired, so it should be working fine.