What Size Berkey For A Family Of Four?

For four people, the Big Berkey is the usual answer and it is a borderline one. It works comfortably for four adults with modest water use, and it starts to feel small for a household with teenagers, a hot climate, or anyone who drinks a lot of tea.

The more useful thing than a model name is the arithmetic, because that tells you whether your four people are at the easy end of four or the difficult end.

Work out your daily volume first

Everyone underestimates this, because they think about drinking water and forget everything else.

What actually goes through a filter in a household that uses one properly:

Drinking. Around 1.5 to 2 litres per adult per day, more in hot weather or with exercise. Children less.

Tea and coffee. Easily a litre a day between four people, often considerably more. This is the item people forget and it is frequently the largest single use after drinking.

Cooking. Pasta, rice, vegetables, stock, soup. A litre or two a day for a family cooking properly.

Water bottles for school and work. Four bottles at half a litre is another two litres, and it all leaves the house in one go in the morning.

Pets. A dog gets through a surprising amount.

Squash, cordials, ice.

Add it up honestly and a family of four using filtered water for everything lands somewhere between 10 and 16 litres a day. Call it 12 as a working figure for four adults, and rather more if two of them are teenagers.

If you only intend to filter drinking water and will use the tap for cooking, the figure drops to perhaps 6 to 8 litres and the sizing question becomes much easier.

The two numbers, and which one matters

Every gravity filter has a tank capacity and a flow rate, and people buy on the first and are limited by the second.

Tank capacity

How much the upper chamber holds. It decides how often somebody has to refill it.

This is the number on the box and the number in the model name, and it is the less important of the two.

Flow rate

How fast the system actually produces filtered water. This is decided by the number of filter elements fitted, not by the size of the tank.

Gravity is the only force pushing water through, and there is not much of it. Each element passes water at its own rate, and two elements produce twice as much as one. A large tank fitted with the minimum number of elements produces water no faster than a small one.

As a rough figure, a pair of elements produces somewhere around 3 to 4 litres an hour. Check the specification for the elements you are buying, because it varies, but that is the order of magnitude.

What that means for four people. Twelve litres a day at 3.5 litres an hour is about three and a half hours of filtering. Spread across a day that is fine. Concentrated into the hour before school, it is not.

The morning problem

This is what actually decides whether a system feels adequate, and it is not on any specification sheet.

Demand is not spread evenly. Between seven and half past eight in the morning, a family of four wants: four water bottles filled, a kettle or two, and glasses with breakfast. That is easily five or six litres inside ninety minutes.

A two-element system produces about five litres in ninety minutes if it starts full and keeps going. So it just about copes — provided somebody filled it the night before.

If the lower chamber was already empty, it does not cope, and somebody goes to work with a bottle of tap water.

The practical answer is a habit rather than a bigger system: fill the upper chamber every evening. That gives the filter all night to work at its own pace, and the morning draws on a full lower chamber rather than waiting for production.

If nobody in the household will reliably do that, buy for the peak rather than the average. That means more elements, or a bigger system that takes them.

Where the Big Berkey sits

Roughly eight and a half litres of tank, and it takes two to four elements.

Fitted with two elements it produces around 3 to 4 litres an hour. Fitted with four it produces around 7 to 8, and the tank capacity is unchanged.

That second sentence is the important one for a family of four. If a Big Berkey feels slow, the answer is usually more elements rather than a bigger system, and it is much cheaper.

Big Berkey with two elements suits: four people filtering drinking water and hot drinks, with someone who reliably refills it in the evening. Around 8 to 10 litres a day.

Big Berkey with four elements suits: four people filtering everything including cooking, with a demanding morning peak. Around 12 to 16 litres a day.

Neither suits: four people who also want a reserve for emergencies, who are on a private supply and filtering everything, or who host regularly.

When to size up

The next size up holds roughly twelve litres and takes up to four elements. Reasons to go there instead:

Teenagers. Two teenagers drink more than two adults, take bottles everywhere, and are entirely unreliable about refilling anything.

A hot climate, or a hot summer. Consumption rises steeply.

Filtering everything. Cooking water included, cordials included, pets included.

A private supply. If a borehole or a spring is your water rather than a mains supply with a filter on top, the filter is doing a much bigger job and you want capacity in hand.

Emergency reserve. A larger lower chamber standing full is drinkable water on hand if the mains fails, which is one of the better reasons to own a gravity filter at all.

Guests, and the household growing. Four becomes five, and a system bought exactly for four has no headroom.

Why not just buy the biggest?

Because oversizing has a real cost, and it is not just money and worktop space.

Growth in these systems is caused by water standing still. A household that empties its lower chamber daily gives bacteria and mould very little opportunity. A household with a system twice the size it needs is holding several days of water permanently, and that is exactly the condition that causes the problem.

This catches people out because the instinct is that more capacity is always better. For this specific issue it is worse. The guide to growth in gravity filters covers what happens and why.

The right size is the one your household actually turns over. For four people that usually means the Big Berkey with enough elements, rather than a larger system running half empty.

The range, and roughly what each holds

Model names tell you the tank size and nothing else, so it is worth having the shape of the range in mind. Check current specifications before buying, because they change.

Travel size, around six litres, two elements. For one or two people, or for taking away. Too small for a family of four in daily use, though useful as a second system for a caravan or a car.

Big, around eight and a half litres, two to four elements. The standard family size and the usual answer for four.

Royal, around twelve litres, two to four elements. The step up, and the sensible choice for five or for a demanding four.

Imperial, around seventeen litres, up to six elements. For larger households, or for four people who want serious reserve capacity.

Crown, around twenty-two litres, up to eight elements. Group and institutional sizes.

Notice that the element count moves with the size, and that is the real difference between them. A Royal fitted with two elements filters no faster than a Big fitted with two. It simply holds more.

This is why comparing models on tank size alone leads people to the wrong purchase. The question is not "how much does it hold" but "how much does it hold, and how fast does it make it".

What it costs to run

Worth knowing before committing, because the running cost of a gravity filter is unusually low and that is a genuine point in its favour.

The main elements are rated by volume, and the figures are large — commonly several thousand litres per pair. A family of four getting through twelve litres a day will take a long time to reach that.

This is the arithmetic that matters: divide the rated volume by your daily use to get the number of days, and remember that a set of four elements lasts roughly twice as long as a set of two, because the work is shared.

There is also a time limit alongside the volume limit, and it exists for microbiological reasons rather than capacity. Even a lightly used element should not stay in a system indefinitely.

Beyond that there is nothing. No electricity, no plumbing, no waste water, no drain connection. Compared with a plumbed system this is the cheapest thing to keep running, and it is one of the reasons these filters suit households that are wary of ongoing costs.

If you also fit fluoride reduction elements, those are a separate consumable on a much shorter cycle, and they are the item that changes the annual cost meaningfully.

Elements: the decision people get wrong

Most systems ship with the minimum number of elements and can take more. This is the cheapest and most effective upgrade available.

Doubling the elements roughly doubles the flow rate. It does nothing to the tank size, and it does not shorten the life of each element — the total volume they can handle between them is unchanged, so a set of four lasts about twice as long as a set of two doing the same work.

The practical effect is that adding two elements to a Big Berkey costs less than upgrading to the next model, takes ten minutes, and solves the flow problem directly rather than solving the tank problem you may not have had.

Rule of thumb: if the complaint is "it runs out", you need a bigger tank or better refilling habits. If the complaint is "it is too slow", you need more elements.

For a family of four, "too slow" is by far the more common complaint.

Practical things to check before buying

Height, including the lid and the filling clearance. These systems are tall, and you need to be able to lift the lid off and pour water in without the wall cupboard in the way. Measure from the worktop to the underside of the cupboard, not just the height of the unit.

Where you will fill it from. If it lives on a worktop away from the sink, somebody is carrying jugs. Next to the sink is better.

Whether the tap clears the worktop. A tall glass under a low tap is a daily irritation. A stand solves it, and is worth buying at the same time.

Weight when full. Twelve litres of water is twelve kilograms plus the unit. Make sure the surface takes it and nobody has to move it full.

Sunlight. Direct sun causes algae. A shaded corner is better than a sunny window.

Element cost over five years. Elements last a long time, but check the price and the rated volume, and work out the annual figure before committing to a system.

A rough decision rule

Four adults, filtering drinking water and hot drinks only, someone refills it in the evening: Big Berkey, two elements.

Four people, filtering everything including cooking: Big Berkey, four elements.

Four including teenagers, or a hot climate, or a private supply, or wanting an emergency reserve: the next size up, with four elements.

Four people who might become five: read the sizing for a family of five before buying, because the threshold is closer than it looks.

The short version

Work out your daily litres honestly, including tea, coffee, cooking and bottles. Four people filtering everything is usually 10 to 16 litres a day.

The Big Berkey holds enough for that. Whether it produces it fast enough depends on how many elements you fit, and two is often not enough for four people with a busy morning. Four elements roughly doubles the flow and costs far less than a bigger system.

Fill it every evening and the morning looks after itself. And do not buy larger than you will empty — standing water is what causes growth in these systems.