Ask someone how old their puppy is, and they will often say “about three months”. Ask the vet, and the answer is more likely “eleven weeks and two days”. The difference is not pedantry. Puppies change so fast that a few weeks can mean a different food amount, a different number of meals, a different vaccination step and a different stage of development. This guide explains how to work out age in weeks, converts weeks to months, lists the care decisions that depend on it and shows how to use age together with weight, which is the other number that drives feeding.
How to work out age in weeks
The calculation is simple:
- Find the puppy’s birth date.
- Count the number of days from that date to today.
- Divide by 7. The whole number is the weeks, and the remainder is the extra days.
A worked example: a puppy was born on 1 August 2026. From 1 August to 1 September is 31 days, to 1 October is 61 days and to 18 October is 78 days. 78 ÷ 7 = 11 remainder 1, so the puppy is 11 weeks and 1 day old. The Puppy & Kitten Age Calculator does the counting and also gives months and years, and the Dog Age from Birth Date Calculator handles older dogs.
Weeks to months
Months are not all the same length, so conversion is approximate. A year has about 52.18 weeks, so a month averages about 4.35 weeks.
| Age in weeks | Approximate months | Notes |
|---|---|---|
| 8 | 1.8 | Typical age puppies go to new homes |
| 10 | 2.3 | |
| 12 | 2.8 | |
| 13 | 3.0 | |
| 16 | 3.7 | |
| 17 | 3.9 | Feeding stage changes in our calculators |
| 20 | 4.6 | |
| 26 | 6.0 | Half a year |
| 39 | 9.0 | |
| 52 | 12.0 | One year |
Use weeks when you talk to the vet or follow a feeding schedule, and months when you chat with friends. If a chart uses months and your puppy is, say, 15 weeks old, convert: 15 ÷ 4.35 = 3.4 months, so use the row for just under 3.5 months, and ask your vet when in doubt.
What changes at which age
Weeks drive several practical decisions.
| Decision | How age matters | Where to look |
|---|---|---|
| Energy factor | 3.0 × resting energy before about 17 weeks, 2.0 × after, in our calculators | Puppy feeding chart |
| Meals per day | Commonly 4, then 3, then 2 as the puppy grows | How many times a day a puppy should eat |
| Vaccinations | A course timed by the vet, with intervals in weeks | Your vet’s schedule |
| Parasite control | Product and timing depend on age and weight | Your vet |
| Socialisation | Early weeks are an important window | Your trainer and vet |
| Crate and divider | Size changes with growth | Puppy crate size |
| Neutering discussion | Timing depends on the dog and the vet’s advice | Your vet |
None of these is a rule that applies identically to every puppy; they are the questions to ask at each visit.
Age and weight together
Age tells you the stage; weight tells you the amount. A 12-week-old puppy can weigh 3 kg or 10 kg depending on breed, so you need both. The Puppy Food Calculator takes the age in weeks and the weight in kilograms: weeks decide the factor (3.0 under 17 weeks, 2.0 after), and weight decides resting energy through 70 × weight ^ 0.75.
| Age (weeks) | Weight | Factor | Calories a day |
|---|---|---|---|
| 12 | 3 kg | 3.0 | 479 |
| 12 | 10 kg | 3.0 | 1,181 |
| 20 | 8 kg | 2.0 | 666 |
| 20 | 15 kg | 2.0 | 1,067 |
Notice that the 12-week-old of 10 kg needs about 2.5 times the calories of the one of 3 kg, though they are the same age. A chart based only on age would give one of them the wrong amount.
Milestones by week: general ideas
Development varies by individual and breed, and vets are the source for specifics. As a very general picture, puppies are with their mother and littermates for the earliest weeks, begin eating solid food around the time their teeth appear, go to new homes at around 8 weeks or later depending on law and practice, begin vaccination courses and early training and continue to socialise and learn for months. Large breeds mature more slowly than small breeds. Keep a log of what your puppy does each week, with photos, since changes are quick.
Tracking your puppy
A simple weekly sheet works well. Choose a weigh-in day, such as Saturday morning, and record:
| Week | Date | Weight (kg) | Daily food (g) | Meals | Notes |
|---|---|---|---|---|---|
| 8 | |||||
| 9 | |||||
| 10 |
When you update the food amount each week, check the age in weeks at the same time so you do not miss the stage change. The age row on the chart should move at least monthly.
If you do not know the birth date
For rescue puppies, the birth date may be unknown. Ask the rescue or breeder for their best estimate, and ask your vet to estimate from the teeth and body. Use a notional birth date such as the day you collect the puppy minus the estimated age, and record it. Be prepared to adjust if your vet revises the estimate. The estimate is good enough for feeding and care. Our guide on how to tell a dog’s age explains what vets look at.
Age in weeks versus corrected age
Some puppies are born early or small, and breeders and vets sometimes talk about their development rather than calendar age. If this applies to your puppy, follow your vet’s advice about food and care. The calculators use calendar age, which suits most puppies.
Common mistakes
- Using months when a chart uses weeks.
- Forgetting to update age as the puppy grows.
- Counting from the day you got the puppy rather than the birth date.
- Using the wrong stage factor for feeding.
- Relying on age alone without weight.
- Assuming all breeds develop at the same speed.
Questions to ask your vet
- How old is my puppy by your estimate?
- When are the next vaccination and parasite-control steps?
- When should we change the number of meals?
- When should we move to adult food?
- When will my puppy stop growing?
Age in weeks at appointments and holidays
Because you can enter any date into the Puppy & Kitten Age Calculator, you can plan ahead. Your puppy was born on 12 July: how old will it be on 15 November? From 12 July to 15 November is 19 + 31 + 30 + 31 + 15 = 126 days, which is 18 weeks exactly. At that age, the feeding stage has changed to 2.0 times resting energy, the meals are likely to be three a day and the puppy may be ready for the next steps your vet sets. Doing this kind of sum in advance lets you prepare food and gear for the stage, not the one you have left.
Age on paper and in real life
A puppy’s age on paper does not always match its development. Large breeds look bigger and sometimes act younger than small breeds of the same age. Puppies from a hard start may be behind. Your vet and trainer look at the individual puppy, and so should you. Use the age in weeks for the calendar of care, and use observation for everything else.
A short glossary
| Term | Meaning |
|---|---|
| Week | Seven days |
| Whole weeks | Days since birth ÷ 7, rounded down |
| Extra days | The remainder |
| Notional birth date | An estimated date used for records when the true date is unknown |
| Feeding stage | The factor used to calculate energy: 3.0 under about 17 weeks, 2.0 after |
| Life stage | AAHA’s puppy, young adult, mature adult, senior and end of life |
Using the same terms with your vet and trainer avoids confusion, particularly at the awkward moments around 16 to 18 weeks when owners say “four months”.
Common questions about age in weeks
My puppy is 100 days old. How many weeks? 100 ÷ 7 = 14 weeks and 2 days.
My puppy is 4 months old. How many weeks? About 17 weeks, since 4 × 4.35 = 17.4. Check the exact date, because the feeding stage changes around then.
How many weeks until my puppy is 6 months? 26 weeks in total. A 10-week-old puppy has 16 weeks to go.
How old is a puppy at 9 months? About 39 weeks. Many small and medium breeds are near adult size by then; large breeds are still growing.
Does the number of days in a month matter? Only for conversions. Counting days is exact.
What age counts for vaccination timing? Your vet’s schedule uses the puppy’s actual age in weeks, so give them the correct birth date.
Keeping a growth and age diary
A diary pairs age with weight and milestones. Photograph your puppy at the same spot every four weeks, with a familiar object for scale. Note the first time it sits on cue, stays alone for a few minutes, walks on a lead, meets a new dog or sleeps through the night. These notes are fun and useful: they help trainers and vets understand how your puppy is developing. The Puppy Food Calculator takes the age in weeks, so keep the number handy.
Age in weeks and the weekly weigh-in
Link your weigh-in day to your puppy’s birthday weekday. If your puppy was born on a Saturday, each Saturday it is a whole number of weeks older. Weigh and measure on that day, and write the age in weeks at the top of the page. This simple trick keeps the age and the weight in sync, and makes the log easy to read. When the age reaches 17, use the second-stage factor.
Quick summary
- Puppy age in weeks = days since birth ÷ 7, whole number plus leftover days.
- A month is about 4.35 weeks, so 26 weeks is about 6 months and 52 weeks is a year.
- Weeks decide feeding stage and meals; weight decides the amount.
- Update the stage when the puppy passes about 17 weeks.
- Keep a weekly log of weight, food and notes.
- Use the Puppy & Kitten Age Calculator for the exact figures.
Frequently asked questions
How do I work out my puppy's age in weeks?
Count the days from the birth date to today and divide by 7. The whole number is the weeks, and the remainder is the extra days.
How many weeks in a month for a puppy?
About 4.3 weeks per month on average, so 8 weeks is just under 2 months and 26 weeks is about 6 months.
Why do vets and feeding guides use weeks for puppies?
Puppies change fast, so weeks give finer detail than months for feeding, vaccinations and development.
What if I do not know my puppy's birth date?
Ask the breeder or rescue, or your vet for an estimate from the teeth and body. Use a notional date and record it.
At what age does a puppy's food amount change?
In our calculators the factor drops from 3.0 to 2.0 times resting energy at about 17 weeks (4 months). Your vet may suggest changes at other times.
How many meals does a puppy need at each age?
Commonly 4 under 13 weeks, 3 from 13 to 25 weeks and 2 from 26 weeks, though your vet may suggest a different pattern.
Is there a calculator for this?
Yes. The Puppy & Kitten Age Calculator gives age in days, weeks, months and years from the birth date.
Sources
- AAHA (2019) Canine Life Stage Guidelines. Journal of the American Animal Hospital Association 55:267-290
- NRC (2006) Nutrient Requirements of Dogs and Cats, National Academies Press
- Calendar arithmetic: age in weeks = whole days since birth ÷ 7
Related calculators
- Puppy & Kitten Age CalculatorFind your puppy's or kitten's exact age in weeks, months and days from its birth date.
- Puppy Food CalculatorCalculate how much a growing puppy needs, how many meals a day, and how much per meal.
- Dog Age from Birth Date CalculatorEnter your dog's birth date and size to see its exact age and its age in human years.
Keep reading
- Puppy Feeding Chart: Calories by Age StageA puppy feeding chart with calories and grams by weight for two stages: under 4 months and 4 months to adult, plus meals per day and how to move to adult food.
- How Many Times a Day Should a Puppy Eat?How many meals a day a puppy needs by age: four under 13 weeks, three to six months, then two. Meal-size examples, timing, toy breeds and when to ask your vet.
- How to Tell a Dog's Age: Teeth, Coat and EyesHow vets estimate an unknown dog's age from teeth, coat, eyes, muscle and movement, why every estimate is a range, and how to record it and convert it.
- How Old Is My Puppy? Weeks, Months and MilestonesFind out how old a puppy is with a known or unknown birth date: counting weeks and months, estimating from teeth, and what age means for food and vet visits.