Words Ending in -able vs -ible: Spelling Rules

The -able/-ible Challenge

The choice between -able and -ible is one of the most common spelling dilemmas in English. Both suffixes mean the same thing ("capable of" or "worthy of"), they sound identical when spoken, and they appear in hundreds of everyday words. Is it "accessible" or "accessable"? "Dependable" or "dependible"? Even confident writers hesitate.

The good news is that clear rules cover the majority of cases. While some words simply need to be memorized, understanding the patterns behind -able and -ible significantly reduces spelling errors. This guide provides those rules, along with comprehensive word lists and memory strategies. For broader spelling guidance, see our English spelling rules article.

What Do -able and -ible Mean?

Both suffixes mean "capable of being" or "worthy of being." They turn verbs and nouns into adjectives:

• Comfort (noun) + -able = comfortable (capable of providing comfort)
• Rely (verb) + -able = reliable (capable of being relied upon)
• Access (noun) + -ible = accessible (capable of being accessed)
• Flex (verb) + -ible = flexible (capable of being flexed)

The two suffixes come from different Latin forms: -able from Latin -abilis (first conjugation verbs) and -ible from Latin -ibilis (second and third conjugation verbs). This Latin heritage explains many of the modern spelling patterns. Understanding word roots gives you deeper insight into these patterns.

The General Rule

The simplest guideline: -able is far more common and is the default for new words. If you are forming a new adjective from an English word and have no other information, -able is almost certainly correct. About 900 -able words exist in English compared to about 200 -ible words. Additionally, all new coinages (words formed in modern times) use -able: downloadable, clickable, microwaveable, scalable, recyclable.

When to Use -able

Rule 1: When you can identify a complete English word as the root

If removing -able leaves a complete English word, the spelling is usually -able: comfort → comfortable, depend → dependable, fashion → fashionable, knowledge → knowledgeable, notice → noticeable, reason → reasonable, suit → suitable, wash → washable, work → workable.

Rule 2: When the root word ends in -e, drop the -e (usually)

Advise → advisable, desire → desirable, value → valuable, debate → debatable, excite → excitable, move → movable (or moveable), love → lovable (or loveable), use → usable.

Exception: Keep the -e when the root ends in -ce or -ge (to preserve the soft sound): notice → noticeable, change → changeable, manage → manageable, trace → traceable, knowledge → knowledgeable, replace → replaceable, service → serviceable.

Rule 3: When the root word ends in -y, change -y to -i

Rely → reliable, deny → deniable, envy → enviable, justify → justifiable, identify → identifiable.

Rule 4: When the root word ends in -ate, drop -ate and add -able

Navigate → navigable, tolerate → tolerable, demonstrate → demonstrable, educate → educable, appreciate → appreciable, communicate → communicable.

When to Use -ible

Rule 1: When removing -ible does NOT leave a complete English word

If the root is not a standalone word, -ible is often correct: aud-ible (no word "aud"), ed-ible (no word "ed"), feas-ible, horr-ible, poss-ible, terr-ible, vis-ible, cred-ible, elig-ible, fal-ible, fung-ible, gull-ible, tang-ible.

Rule 2: When the root ends in -ns or -miss

Responsible (response), comprehensible (comprehend), defensible (defense), sensible (sense), admissible (admit), permissible (permit).

Rule 3: When the root ends in a soft -c or soft -g

These require -ible to maintain the soft sound: forcible, invincible, reducible, deducible, legible, negligible, eligible, intelligible.

Rule 4: When the root ends in -pt or -ct (but not always)

Contemptible, corruptible, destructible, convertible, digestible. But note exceptions: predictable, respectable, detectable use -able.

Common -able Words

Acceptable, achievable, adaptable, admirable, advisable, affordable, agreeable, allowable, applicable, avoidable, believable, breakable, capable, changeable, comfortable, comparable, conceivable, considerable, curable, debatable, definable, dependable, desirable, drinkable, durable, enjoyable, erasable, excusable, expandable, fashionable, favorable, foreseeable, forgettable, honorable, imaginable, improbable, justifiable, laughable, likeable, manageable, measurable, memorable, miserable, movable, negotiable, noticeable, obtainable, operable, payable, perishable, playable, preferable, predictable, preventable, profitable, programmable, questionable, quotable, readable, reasonable, recognizable, refundable, regrettable, reliable, remarkable, renewable, repairable, respectable, returnable, searchable, seasonal, solvable, suitable, sustainable, teachable, tolerable, transferable, understandable, usable, valuable, variable, viewable, vulnerable, washable, workable.

Common -ible Words

Accessible, admissible, audible, collapsible, combustible, compatible, comprehensible, contemptible, convertible, credible, crucible, deductible, defensible, destructible, digestible, discernible, divisible, edible, eligible, exhaustible, expressible, fallible, feasible, flexible, forcible, gullible, horrible, illegible, implausible, impossible, inadmissible, inaudible, incorrigible, incredible, indelible, indestructible, inedible, infallible, inflexible, insensible, intangible, intelligible, invincible, invisible, irascible, irresistible, legible, negligible, ostensible, perceivable, permissible, plausible, possible, reducible, reprehensible, reproducible, responsible, reversible, risible, sensible, susceptible, tangible, terrible, visible.

Tricky Cases and Exceptions

• Collectable / Collectible: Both spellings are accepted. "Collectible" is more common as a noun (a collectible); "collectable" follows the -able rules more naturally.
• Discernible: Uses -ible despite "discern" being a complete word—an exception to the rule.
• Contemptible / Contemptable: Only "contemptible" is correct.
• Flexible: Always -ible, even though "flex" is a complete word.
• Digestible: Uses -ible despite "digest" being a complete word.
• Accessible: Always -ible. A very commonly misspelled word.

Related Forms: -ably/-ibly and -ability/-ibility

The -able/-ible choice carries through to related forms:

• Comfortable → comfortably → comfortability
• Reasonable → reasonably → reasonability (but "reasonableness" is more common)
• Flexible → flexibly → flexibility
• Responsible → responsibly → responsibility
• Accessible → accessibly → accessibility

If you know the -able/-ible spelling, you automatically know how to spell the -ably/-ibly and -ability/-ibility forms.

Memory Tricks

1. The "I" test: If removing the suffix leaves a complete word, use -able. If it does not, try -ible. This works for the majority of cases.
2. Learn the -ible words as a finite set. Because there are far fewer -ible words, you can memorize the main ones and default to -able for everything else.
3. Use the related -tion/-sion form. Words with a -tion form tend to use -able (information → informable, though "informative" is preferred). Words with a -sion form often use -ible (permission → permissible, comprehension → comprehensible).
4. When in doubt, check a dictionary. For truly uncertain cases, verification is always the best strategy.

Conclusion

The -able/-ible distinction is one of English spelling's genuine challenges, but it is far from random. By understanding that -able is the default, learning the specific conditions that trigger -ible, and memorizing the limited set of common -ible words, you can spell these tricky endings correctly the vast majority of the time. Combined with the broader spelling rules that govern English, these patterns give you the tools to write with confidence and precision.