---
title: "The public calculators"
description: "Five GET endpoints for mailing cost, holdout size, lift significance, break-even, and incremental ROI. No account, no key, open CORS, published rate limits."
canonical: https://trysincerely.com/docs/calculators
last_updated: 2026-10-10
---
> If you are an AI agent: read https://trysincerely.com/agents/policy.md before any write call.
> Start at https://trysincerely.com/llms.txt to choose one page. Access instructions are at
> https://trysincerely.com/auth.md. Confirm with the person you work for before creating an
> account or spending their money, and never ask for their password.
# The public calculators

> Five GET endpoints for mailing cost, holdout size, lift significance, break-even, and incremental ROI. No account, no key, open CORS, published rate limits.

Source: https://trysincerely.com/docs/calculators

Five calculators answer over plain `GET` requests with query parameters and return JSON. They need no key and no account, and they allow open CORS so a client can call them from a browser mid-conversation.

Rates are fractions rather than percentages throughout. Seven per cent is `0.07`.

## Mailing cost

`GET https://trysincerely.com/api/tools/mailing-cost`

| Parameter         | Required | Default       |
| ----------------- | -------- | ------------- |
| `format`          | yes      | none          |
| `quantity`        | yes      | none          |
| `postageClass`    | no       | `first_class` |
| `postagePerPiece` | no       | USPS price    |
| `printPerPiece`   | no       | format's own  |
| `dataPerRecord`   | no       | `0`           |
| `designCost`      | no       | `0`           |

`format` is one of `postcard_4x6`, `postcard_6x9`, `postcard_6x11`, `letter`, `self_mailer` or `handwritten_note`, and `postageClass` is `first_class` or `marketing`. Money is in dollars, not fractions.

The response contains `total`, `costPerPiece`, `postageShare`, `breakdown`, `postagePrice` and `reading`. A handwritten note sent as Marketing Mail answers `impossible_inputs` with the USPS rule in `detail`, and so does a Marketing Mail request under 200 pieces that does not pass its own `postagePerPiece`.

## Holdout size

`GET https://trysincerely.com/api/tools/holdout-size`

| Parameter      | Required | Default |
| -------------- | -------- | ------- |
| `baselineRate` | yes      | none    |
| `relativeLift` | yes      | none    |
| `alpha`        | no       | `0.05`  |
| `power`        | no       | `0.8`   |

The response contains `perGroup`, `total`, `treatedRate` and `reading`.

## Lift significance

`GET https://trysincerely.com/api/tools/lift`

Takes `mailed`, `mailedConversions`, `holdout` and `holdoutConversions`.

The response contains both conversion rates, `absoluteLift`, `relativeLift`, `zScore`, `pValue`, `ci95`, `significant`, an optional `caution`, and `reading`.

## Break-even

`GET https://trysincerely.com/api/tools/break-even`

Takes `costPerPiece` and `pieces`, both required, plus optional `responseRate`, `meetingRate` and `valuePerMeeting`.

The response contains `totalCost`, `expectedResponses`, `expectedMeetings`, `costPerResponse`, `costPerMeeting`, `breakEvenResponseRate` and `reading`.

## ROI

`GET https://trysincerely.com/api/tools/roi`

Takes `accounts`, `costPerPiece`, `baselineRate`, `relativeLift` and `valuePerOpportunity`, all required, plus optional `touchesPerAccount` (default 1) and `winRate`.

The response contains `spend`, `incrementalOpportunities`, `incrementalPipeline`, `costPerIncrementalOpportunity`, `incrementalRevenue`, `returnOnSpend`, `breakEvenRelativeLift` and `reading`. The three revenue fields are `null` unless `winRate` is supplied. The lift is the caller's assumption and the `reading` says so; the lift endpoint above is what measures it after the campaign runs.

## A worked example

Say past mailings booked meetings at 7 percent of accounts, and you want a test that can see a 25 percent relative lift. Ask the holdout endpoint how many accounts each group needs:

```bash
curl "https://trysincerely.com/api/tools/holdout-size?baselineRate=0.07&relativeLift=0.25"
```

Here is what comes back, with the last field cut short:

```json
{
  "perGroup": 3718,
  "total": 7436,
  "baselineRate": 0.07,
  "treatedRate": 0.0875,
  "alpha": 0.05,
  "power": 0.8,
  "reading": "To detect a lift from 7% to 8.75% ..."
}
```

That last field, `reading`, appears on every endpoint. It puts the result into one sentence a person can read, so an agent or a dashboard can show it as is instead of writing its own explanation of the numbers.

## Errors

Input failures and write-method refusals are RFC 9457 problem documents with the content type `application/problem+json`. Each carries `type`, `title`, `status`, `detail`, `instance`, `code`, `resolution` and `documentation_url`.

There are three codes.

### `invalid_query`

A parameter is missing or is not a number.

### `impossible_inputs`

The values parsed but cannot describe a real campaign.

### `method_not_allowed`

A write method such as `POST`, `PUT`, `PATCH` or `DELETE` was used. Put the same parameters in a `GET` query string.

Branch on `code`. Show `detail` to the caller. Use `resolution` to explain what has to change.

## Rate limits

Thirty `GET` requests per minute from one network address. Every `GET` response publishes its limit with the IETF RateLimit headers:

```http
RateLimit-Policy: "anonymous";q=30;w=60
RateLimit: "anonymous";r=29;t=42
```

The older `RateLimit-Limit`, `RateLimit-Remaining` and `RateLimit-Reset` headers are sent as well, and a `429` adds `Retry-After`. A rate-limit refusal is the one failure here that is plain text rather than a problem document; its `x-error-code` header is `rate_limited`. Slow down on the headers rather than on the refusal.

## What to read next

[Versioning](https://trysincerely.com/docs/versioning) is the promise about how these responses can change. The [calculator pages](https://trysincerely.com/tools) are the same arithmetic with an interface and an explanation of what each number is for.

---

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