Baby age calculator
See your baby's age the way clinics count it: a baby born on 2025-01-15 is 79 weeks and 3 days old on 2026-07-25, which is also 18 months and 10 days. Both figures come from the same two dates. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Birth date
- 2025-01-15
- Reference date
- 2026-07-25
- Weeks
- 79
- Leftover days
- 3
- Months
- 18
- Days
- 10
A baby age calculator answers the question parents type every week: how old is my baby in weeks, and how many months is that? Pediatric visits, vaccine schedules, sleep regressions, and milestone charts all talk in completed weeks at first and then in completed calendar months. This tool takes an explicit birth date and an explicit reference date as civil YYYY-MM-DD values, never invents today from a host clock, and returns both measures from the same pair.
What baby age in weeks and months means
Pediatric care talks about a baby's age in two units that do not stay interchangeable. For the first months, clinics, vaccine schedules, and sleep-training books count completed weeks from the birth date. After that stretch, growth charts, clothing sizes, and well-child visits switch to completed calendar months.
A week is seven civil days, so the week count is the whole-number quotient of the day span between the birth date and the reference date. A calendar month is not thirty days and is not four weeks: it is the count of month anniversaries that have already occurred, with leftover days measured from the last anniversary.
Subtracting years and months on a civil calendar, then stepping back one month when the day-of-month has not yet arrived, is the same contract intake forms use when they ask how many months old a child is. This engine never samples a host clock, a time zone, or daylight-saving tables. Both the birth date and the reference date must be supplied, so the same pair always yields the same weeks and the same months.
A child born on the fifteenth of January two thousand twenty-five is eighteen months and ten days old on the twenty-fifth of July two thousand twenty-six, and that span is also seventy-nine weeks and three days. The same child is six weeks old on the twenty-sixth of February two thousand twenty-five. Leap-day births follow a Gregorian last-day-of-month clamp when months are added: a baby born on the twenty-ninth of February reaches the next month on the last day of a shorter month.
Use this page when the search is baby age in weeks or months; use a whole-year age tool when the question is only completed years of life.
Where a baby age calculator earns its keep
Parenting apps and baby trackers need a single source of weeks and months so a dashboard, a vaccine reminder, and a milestone card never disagree about how old the child is on a chosen day. Clinics and care-coordination tools can print the same numbers on educational handouts while still routing medical decisions to licensed staff. Product teams that size diapers, formula, or clothing by age cohort can snapshot a birth date against a catalog date without hard-coding calendar math in every client.
Keep expectations honest: this capability measures Gregorian completed weeks and completed calendar months between two explicit dates; it does not invent today from a server clock, apply corrected gestational age for premature infants, skip weekends, or convert lunar ages. Pair it with a whole-year age tool when the product copy asks only how old someone is in years, and with a feeding-schedule helper when the next question is how often a newborn typically eats.
What you can do with it
Parenting apps and baby trackers
Show caregivers completed weeks and calendar months next to a log, vaccine reminder, or milestone card from a stored birth date and a chosen snapshot day.
How old is my baby in weeks
Enter a birth date and a reference date to see completed weeks with leftover days, then the matching completed months, without host-clock drift.
Clinic handouts and age-banded products
Snapshot a birth date against a visit or catalog date so educational packets and size charts use the same weeks-and-months pair every time.
FAQ
How does this baby age calculator count weeks and months?
Weeks are floor of the civil day span divided by seven. Months are completed calendar months with leftover days after the last month anniversary.
What if the birth date is after the reference date?
Age cannot be negative; correct the pair rather than expecting a signed week or month count.
Does this use corrected gestational age for premature babies?
No. The engine uses the civil birth date and the reference date only. Apply any corrected-age adjustment yourself before calling if your workflow needs it.
What does the baby age calculator cost?
The browser tool is free on your device.
Why is a month not simply four weeks?
Because a calendar month counts month anniversaries since the birth date, and most months are longer than four weeks. Dividing weeks by four would overstate the age in months, so both figures are worked out separately.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/life/baby-age \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"birth_date":"2025-01-15","reference_date":"2026-07-25"}'const res = await fetch("https://api.kit.forhosting.com/life/baby-age", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"birth_date": "2025-01-15",
"reference_date": "2026-07-25"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/life/baby-age",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"birth_date": "2025-01-15",
"reference_date": "2026-07-25"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/life/baby-age", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"birth_date":"2025-01-15","reference_date":"2026-07-25"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"birth_date":"2025-01-15","reference_date":"2026-07-25"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/life/baby-age", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"birth_date": "2025-01-15",
"reference_date": "2026-07-25"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "life.baby_age",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
The same two dates always produce the same weeks and months in a browser, in continuous integration, and on the API. A birth date that falls after the reference date fails with invalid_input and is not billed. Successful API calls use the published $0.002 base price.
Input contract, calendar validation, and the response shape
Each value should be a civil calendar day in YYYY-MM-DD form, for example two thousand twenty-five hyphen zero one hyphen fifteen for a birth date and two thousand twenty-six hyphen zero seven hyphen twenty-five for the snapshot day. You may prefix a full ISO-8601 datetime; if a time is present only the calendar date part is used, because baby age is a date-to-date measure, not a clock duration with time zones. Years range from one through nine thousand nine hundred ninety-nine, months must be one through twelve, and the day must exist in that month under Gregorian leap rules, including February twenty-nine only in leap years. POST a compact JSON body with two required string fields: birth_date and reference_date. Optional aliases birth, birthdate, dob, and from map to the birth field; reference, as_of, to, and on map to the reference field so form builders and SDKs can keep familiar names. Impossible pairs such as two thousand twenty-three hyphen zero two hyphen twenty-nine are rejected with invalid_input instead of being clamped into March. If the birth date falls strictly after the reference date, the request fails with a clear message that age cannot be negative. Missing fields, empty strings, and non-string values fail with invalid_input naming the field and are not billed.
Where a baby age calculator earns its keep
Call the prepaid API when trackers, reminder jobs, or export pipelines need a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after the engine rejects the request.
Technical questions
Does the baby age calculator use today's date automatically?
No. reference_date is required. The engine never reads the system clock, so results stay identical in CI, browsers, and API workers.