MEI stands for Microempreendedor Individual, the individual microentrepreneur. In practice it is a person who registered a CNPJ and is enrolled in SIMEI, the simplified tax regime inside Simples Nacional. Being a MEI is not a company type or a size: it is a tax enrollment. Two things are often confused. Every MEI is enrolled in Simples Nacional, but not every Simples Nacional company is a MEI. And the ME size (microempresa) is a separate classification: a company can be ME without being a MEI, and a MEI shows up in the registry with ME size.
The Federal Revenue records the enrollment on the Simples Nacional portal, under "Consulta Optantes". Given a CNPJ, it shows whether the company is enrolled in Simples Nacional and whether it is enrolled in SIMEI, with the opt-in date and, if any, the exclusion date. The Cartão CNPJ (registration certificate) does not carry this information. It shows legal nature, size and registration status, but not whether the company is a MEI.
Some registry data suggests a MEI: legal nature 213-5 (Empresário Individual), ME size, no partners, and a legal name that starts with the CNPJ digits followed by the person's name, a common pattern in recent registrations. None of these clues confirms the enrollment. An individual entrepreneur may have stopped being a MEI and kept the same legal nature and legal name. Only the SIMEI record confirms it.
On the public page of any CNPJ at cnpj-api.com, the Simples Nacional block shows "MEI: Sim" or "MEI: Não". In the API, the /v1/cnpj response carries a top-level mei field: true when the company is enrolled in SIMEI and false otherwise, including when it never opted into Simples Nacional. The field is always present, so you can filter on it without checking whether other fields exist. For the dates, use empresa.simei, which carries optante, data_opcao and data_exclusao. The /v1/simples endpoint returns only that part of the response, with the same fields.
Disenrollment happens when the company exceeds the MEI revenue limit, hires more than one employee, opens a branch, admits a partner or starts an activity that is not allowed. The company still exists, often as an ME inside Simples Nacional, but it is no longer a MEI. In a lookup this shows as simei.optante false with a filled data_exclusao, and the mei field becomes false. The opt-in and exclusion history stays visible on Consulta Optantes.
No. ME is the microenterprise size, which applies to any company under the revenue limit. A MEI shows up with ME size, but most MEs are not MEIs.
No. MEI is an enrollment inside Simples Nacional. ME and EPP companies can also be enrolled in Simples without being a MEI.
No. The certificate shows legal nature, size and registration status, but not the SIMEI enrollment. Use Consulta Optantes on the Simples Nacional portal or the API.
Yes. The number format does not change the enrollment. The check is the same: the mei field in the API or Consulta Optantes.