What is comprehensible input?

Comprehensible input is language in your target language that you can understand even when you do not know every word or grammatical structure.

It gives you enough meaning to follow what is being said or written while still exposing you to something new. The idea is closely associated with linguist Stephen Krashen’s input hypothesis: learners develop their language ability when they understand input containing features just beyond their current competence—often summarized as i + 1.

What makes input comprehensible?

Previous knowledge and context help you make sense of unfamiliar language.

In conversation, context can include a speaker’s gestures, facial expressions, surroundings, and the situation itself. At a café, for example, the setting narrows the range of things someone is likely to say.

In writing, the topic, format, illustrations, headings, and words you already know all provide clues. A sports report and a restaurant menu prepare you for very different vocabulary before you read the first sentence.

What does i + 1 mean?

In the shorthand i + 1, i represents a learner’s current competence and + 1 represents language that contains something new. It is a conceptual description, not a precise formula or a requirement that every text contain exactly one unknown feature.

Research on reading suggests that knowing roughly 95% of the words in a text may allow reasonable comprehension, while closer to 98% is often needed for comfortable independent reading. These figures are useful guidelines rather than a universal test: background knowledge, repetition, visual support, and the difficulty of the unknown words also matter.¹

How is it connected to compelling input?

Compelling input is input that holds your attention so strongly that the language-learning activity recedes into the background. Comprehensibility makes input possible to follow; compelling subject matter makes you want to continue.

Self-selected reading can provide both. Researcher Beniko Mason has described how sustained self-selected reading can generate motivation rather than merely depend on motivation that already exists.²

A metaphor from ancient Greece

Comprehensible input is sometimes compared with the story of Milo of Croton. The ancient Greek athlete was said to have trained by carrying a calf as it gradually grew into a bull. In the same way, learners can encounter increasingly challenging language while retaining enough support to understand it.