A guinea pig eats roughly 6 grams of food per 100 grams of body weight each day, with 85–90% of that intake coming from hay and grass. In practical terms, that means an adult guinea pig needs an unlimited supply of hay available around the clock, about one-eighth of a cup of pellets, around half a cup of fresh vegetables, and 80–100 ml of water daily.
Hay: The Bulk of the Diet
Hay and grass should make up 85–90% of everything your guinea pig eats. Unlike pellets or vegetables, you genuinely cannot overfeed hay. Guinea pigs are natural grazers, and you’ll see them munching on hay throughout the day and night. This constant chewing serves two purposes: it keeps their continuously growing teeth worn down to a healthy length, and it provides the fiber their digestive systems need to function properly.
Timothy hay or orchard grass are the best choices for adult guinea pigs. These grass hays provide adequate nutrition without excess calories. Young guinea pigs under six months, along with pregnant or nursing mothers, benefit from alfalfa hay mixed in with their grass hay. Alfalfa is more calorie-dense and provides the extra energy growing or reproducing bodies need. Once a guinea pig reaches adulthood, though, alfalfa should only be an occasional treat, not a staple. The exception is senior guinea pigs who are losing weight and could use the caloric boost.
Keep the hay rack full at all times. If your guinea pig ever runs out, something needs to change. A single guinea pig can easily go through a large handful of hay every day.
Pellets: A Small Daily Supplement
Pellets play a supporting role in the diet, not the starring one. An adult guinea pig needs roughly one-eighth of a cup (about one to two tablespoons) of plain, timothy-based pellets per day. Choose pellets formulated specifically for guinea pigs, because these are fortified with vitamin C, a nutrient guinea pigs cannot produce on their own.
Give a fresh portion each morning rather than topping off yesterday’s leftovers. Vitamin C breaks down with air exposure, so stale pellets lose their nutritional value quickly. Check the best-before date on the bag for the same reason. Avoid muesli-style mixes with seeds, dried fruit, or colorful bits. Guinea pigs will pick out the sugary pieces and leave the nutritious ones behind.
Fresh Vegetables: About Half a Cup
Fresh vegetables and leafy greens provide vitamin C and variety, but they should remain a small part of the overall diet. Around 50 grams, or roughly half a cup, is the daily target per guinea pig. Offer a mix of two or three different vegetables each day to keep the nutrient profile balanced.
Good everyday options include bell peppers (especially red and yellow, which are packed with vitamin C), romaine lettuce, cucumber, and zucchini. You can rotate in herbs and leafy greens a few times a week for variety. However, some popular greens are surprisingly high in calcium and can contribute to bladder stones if fed too often. Kale, spinach, watercress, rocket, spring greens, parsley, and dandelion greens all fall into this high-calcium category. Herbs like basil, dill, rosemary, and thyme are even higher. These should be avoided as regular staples or given only in very small amounts on rare occasions.
Medium-calcium vegetables like cabbage and coriander can be offered occasionally without much concern, but they shouldn’t appear in the bowl every single day.
Fruit: A Rare Treat Only
Fruit is high in sugar, and guinea pigs don’t need much of it. A small slice of apple, a couple of blueberries, or a thin wedge of strawberry once or twice a week is plenty. Think of fruit the way you’d think of dessert: fine in small amounts, problematic as a habit. Too much sugar contributes to obesity and can upset digestion.
Vitamin C Needs
Guinea pigs, like humans, cannot manufacture their own vitamin C. Research on guinea pig nutrition found that healthy young animals need about 5 mg of vitamin C per 100 grams of body weight each day. For a typical adult weighing around 1,000 grams, that works out to roughly 50 mg daily. Stressed, sick, or pregnant guinea pigs need even more.
A combination of fortified pellets, bell peppers, and leafy greens usually covers this requirement. If your guinea pig is recovering from illness or seems under the weather, a liquid vitamin C supplement designed for guinea pigs can help bridge the gap. Avoid vitamin C drops added to the water bottle, since the vitamin degrades quickly in water and most guinea pigs drink less when the taste changes.
Water Intake
Each guinea pig needs a minimum of 80–100 ml of fresh water per day. A standard small-animal water bottle holds about 250–350 ml, so for a single guinea pig you’ll likely refill it every two to three days, but you should swap in fresh water daily regardless. Some guinea pigs prefer a heavy ceramic bowl over a bottle. Either works as long as the water stays clean. Guinea pigs eating lots of fresh vegetables may drink slightly less from their bottle, but clean water should always be available.
Adjusting for Size and Life Stage
Healthy adult males typically weigh between 900 and 1,200 grams, while females usually range from 700 to 900 grams. A larger boar at the top of that range will naturally eat more hay and drink more water than a petite sow, so don’t worry if intake varies between your animals.
Young guinea pigs under six months are still growing and benefit from slightly more pellets and the addition of alfalfa hay alongside their regular grass hay. Pregnant and nursing mothers also need extra calories and calcium during this period. Once babies are weaned and reach about six months of age, transition them fully to timothy hay and reduce pellets to the standard adult portion. Weighing your guinea pig weekly on a small kitchen scale is the simplest way to confirm they’re eating the right amount. Gradual weight loss or sudden gain over a few weeks is worth investigating, even if your feeding routine hasn’t changed.