diff --git a/README.md b/README.md
index 482d636..9eaf235 100644
--- a/README.md
+++ b/README.md
@@ -75,6 +75,12 @@ See [docs/INDEXER.md](./docs/INDEXER.md) for a full table of which on-chain
 events the indexer handles, which tables each one writes, and how the
 checkpoint and out-of-window recovery work.
 
+## API statistics
+
+See [docs/STATS.md](./docs/STATS.md) for the definitions of NGO and platform
+amounts, stream counts, donor counts, cancellation handling, and
+`platformSharePercent`.
+
 ## Notification events
 
 When `NOTIFY_WEBHOOK_URL` is set, the indexer POSTs a JSON body to that URL
diff --git a/docs/STATS.md b/docs/STATS.md
new file mode 100644
index 0000000..6919137
--- /dev/null
+++ b/docs/STATS.md
@@ -0,0 +1,71 @@
+# NGO and platform statistics
+
+The API reports stream amounts in the token's smallest (base) units. Amount
+fields are decimal strings, not JavaScript numbers. Counts are JSON numbers.
+The definitions below apply to both NGO detail endpoints:
+
+- `GET /ngos/:id`
+- `GET /impact/:ngoId`
+
+## Per-stream accounting
+
+A stream's contribution depends on its status:
+
+| Stream status | Amount counted as committed |
+| ------------- | --------------------------- |
+| `ACTIVE`      | `balance + withdrawn`       |
+| `CANCELLED`   | `withdrawn` only            |
+
+When a stream is cancelled, the contract settles the accrued amount to the NGO
+and refunds the remaining `balance` to the donor. The database then records a
+zero balance, so counting that balance would incorrectly include refunded
+funds. `withdrawn` includes amounts withdrawn during the stream and the amount
+settled during cancellation.
+
+## `GET /ngos/:id` stats
+
+The `stats` object contains:
+
+- **`totalCommitted`** — the sum of each stream's committed amount using the
+  status rules above. It includes active funds that remain in a stream because
+  they are committed for future withdrawal.
+- **`totalWithdrawn`** — the sum of `withdrawn` for every stream belonging to
+  the NGO, including cancelled streams.
+- **`activeStreamCount`** — the number of the NGO's streams whose status is
+  `ACTIVE`. Cancelled streams are not included.
+- **`donorCount`** — the number of distinct donors with a stream for this NGO.
+  Multiple streams from one donor count once.
+
+## `GET /impact/:ngoId` stats
+
+This endpoint reports the same amount semantics with impact-oriented names:
+
+- **`totalCommitted`** and **`totalWithdrawn`** have the same definitions as
+  the fields on `/ngos/:id`.
+- **`activeStreams`** — the number of `ACTIVE` streams for this NGO.
+- **`cancelledStreams`** — the number of `CANCELLED` streams for this NGO.
+- **`uniqueDonors`** — the number of distinct donors with a stream for this
+  NGO.
+- **`platformSharePercent`** — this NGO's committed amount divided by the
+  committed amount for every stream on the platform, multiplied by 100:
+
+  ```text
+  ngoCommitted / platformCommitted * 100
+  ```
+
+  The result is expressed as a percentage with at most two decimal places.
+  The current implementation truncates, rather than rounds, beyond two
+  decimal places. If the platform has no committed amount, the value is `0`.
+
+For example, if an NGO has `1,200` committed units and the platform has `2,000`,
+its `platformSharePercent` is `60`.
+
+## `GET /impact` platform totals
+
+The platform-wide endpoint uses the same cancellation rule and returns:
+
+- **`totalCommitted`** — committed amount across all streams.
+- **`totalWithdrawn`** — withdrawn amount across all streams.
+- **`activeStreams`** — number of active streams across the platform.
+- **`verifiedNgoCount`** — number of NGOs whose on-chain `verified` flag is true;
+  this is independent of whether an NGO currently has streams.