Which US industries actually run Microsoft 365? We measured 13,051 businesses

The published figures for Microsoft 365 adoption are global seat counts or vague groupings — “finance, legal and the public sector”. We could not find anyone publishing it by trade, which is the number you need if you are deciding whether a tool that only works with Microsoft 365 is worth building, buying or selling. If you know of a source that does, we would genuinely like to see it.

So we measured it. Every business below is a real US company with its own domain. The mail platform comes from public DNS records, not a survey and not a vendor report.

The results

Across all 13,051 businesses, 50% run Microsoft 365. The ± figures are 95% confidence intervals; the smallest trade sample is 286 businesses, so even the widest is ±5.2 points. The gap between the top and the bottom of the table is 42 points — far outside the error bars, and far larger than we expected.

What the split is actually about

The ordering is not random, and it is not about company size. It tracks what a firm sells. Businesses that sell professional judgement — staffing, engineering, financial advice, accounting, law, consulting, IT services — cluster between 56% and 72%. Businesses that sell presentation — advertising and design at 30%, estate agency at 39% — sit at the bottom, and they are the only two trades where Google Workspace is close to or ahead of Microsoft.

Advertising and design is the clearest case: 51% on Google against 30% on Microsoft, the only inversion in the table. Creative studios choose the tools their clients and collaborators already use for shared documents, and that has been Google for a decade. Regulated and billable-hour professions went the other way, and stayed.

How this was measured

The sample. 16,148 US businesses that publish a website, taken from OpenStreetMap, which tags a business by what it does (office=lawyer, office=employment_agency, and so on). 13,051 of those domains returned a usable DNS answer and form the denominator; the rest resolved to nothing and are excluded rather than counted either way.

The test. A domain counts as Microsoft 365 if any of three public signals matches: an MX record ending in .mail.protection.outlook.com, an SPF record including spf.protection.outlook.com, or an autodiscover record pointing toward outlook.com. Checking all three matters: a large share of Microsoft tenants sit behind a security gateway such as Proofpoint or Mimecast, so their MX points at the gateway and an MX-only test would score them as “other” — and those tend to be the larger organisations.

Corrected 23 September 2026. The first version of this page also counted a fourth, weaker signal: a Microsoft domain-verification TXT record on its own. That record shows a domain was once verified in a Microsoft tenant, not where its mail runs, and 111 of the 271 domains it alone had classified turn out to receive mail at Google. Every figure here was recomputed without it. The ranking barely moved; the biggest change was financial advice, from 67% to 59%.

Where this is weak

The limits matter for how far you can push these numbers:

• This is not a random sample of US businesses. It is businesses that someone mapped in OpenStreetMap and that publish a website. Both skew toward established firms with a physical address.
• It counts domains, not seats or people. A 20-person firm and a 2,000-person firm count once each, so this is not a market-share figure.
• A domain on Microsoft 365 does not mean everyone uses Outlook. It means the organisation’s mail runs there.
• Trades are as OpenStreetMap tags them, and tagging is uneven by region.

Business data from OpenStreetMap contributors, used under the Open Database Licence. Measured September 2026. If you want the per-trade breakdown behind a figure, ask and we will send it.

Why we were counting in the first place

OpenSlot reads Microsoft 365 free/busy and turns it into a paste-ready list of times for an email, so a firm that is not on Microsoft 365 cannot use it. We needed to know which trades were worth talking to. The answer turned out to be more interesting than the question, so here it is. The trade at the top of the table is also the one that schedules hardest: sending interview availability to candidates is the workflow this was originally built for.

Common questions