Synonyms for "useless" include:

• Worthless: Having no value or worth.

• Ineffective: Not producing the desired result.

• Futile: Producing no result or effect; vain.

• Pointless: Lacking purpose or meaning.

• Unproductive: Not yielding useful results.

• Impractical: Not suitable for practical use.

• Inoperable: Not functioning or working.

• Unusable: Not capable of being used.

• Unavailing: Not producing the desired effect.

• Inutile: Not useful or valuable.

Formal synonyms for "useless" include:

• Ineffectual

• Inept

• Incompetent

• Unserviceable

• Unworkable

Informal or slang terms include:

• No good

• Good for nothing

• Pants (UK slang)

• Rubbish (UK informal)

These synonyms vary slightly in nuance—some emphasize lack of function, others lack of value or skill—so context determines the best fit.