100 :continue
101 :switching_protocols
102 :processing
200 :ok
201 :created
202 :accepted
203 :non_authoritative_information
204 :no_content
205 :reset_content
206 :partial_content
207 :multi_status
226 :im_used
300 :multiple_choices
301 :moved_permanently
302 :found
303 :see_other
304 :not_modified
305 :use_proxy
307 :temporary_redirect
400 :bad_request
401 :unauthorized
402 :payment_required
403 :forbidden
404 :not_found
405 :method_not_allowed
406 :not_acceptable
407 :proxy_authentication_required
408 :request_timeout
409 :conflict
410 :gone
411 :length_required
412 :precondition_failed
413 :request_entity_too_large
414 :request_uri_too_long
415 :unsupported_media_type
416 :requested_range_not_satisfiable
417 :expectation_failed
422 :unprocessable_entity
423 :locked
424 :failed_dependency
426 :upgrade_required
500 :internal_server_error
501 :not_implemented
502 :bad_gateway
503 :service_unavailable
504 :gateway_timeout
505 :http_version_not_supported
507 :insufficient_storage
510 :not_extended
You can also do Rack::Utils::HTTP_STATUS_CODES which has the mapping of status codes and their definition.