Does a large reed diffuser give more scent, or does it simply last longer

In short

• Bottle volume sets how long it lasts; the number, thickness and exposed length of the reeds set how much scent reaches the room.
• A large bottle belongs in a hallway, on a landing or in an open-plan room with many reeds; in a small loo or a bedroom it is wasted.
• Per millilitre a big bottle is often cheaper, but an electric diffuser with a timer pushes scent across a large room and switches off at night.

What changes with a bigger bottle, and what does not

The bottle is a reservoir. Make it bigger and the reeds keep wicking for longer before the level drops out of reach, but nothing about the glass pushes scent into the room. A litre bottle with three reeds behaves like a small bottle with three reeds, only for longer.

Two things decide the throw. The first is the wet surface standing in the air: more reeds, thicker reeds and more length above the neck mean more fragrance evaporating at once. The second is the liquid, a fragrance thinned with a light base; the thinner the base, the faster it climbs and leaves.

How many reeds, and how thick, for a big bottle

A large bottle usually has a wider neck, and the neck is the budget. Stand in as many reeds as fit without touching, spread so each has air around it; pressed together they wick as one bundle and the scent stays beside the glass.

Thickness matters more in a tall bottle. A thick reed carries more liquid, offers more surface to the air and stands upright where a thin one bows. It must also reach the base and stand well clear of the top; reeds cut for small bottles are often too short.

Where a large reed diffuser belongs, and where it is wasted

• Hallway: the natural home. People pass within arm's reach, the front door stirs the air, and a tall bottle on a console sits at nose height.
• Landing or stairwell: warm air rising up the stairs carries the scent, and a half-landing shelf keeps the bottle from small hands.
• Open-plan living room: only with the neck full of thick reeds, and even then expect scent around the bottle rather than across the floor.
• Wasted in a downstairs loo or box room, which a small bottle already fills; in a bedroom, where a faint trickle is enough; and beside a radiator or open window.

The honest expectation for a big room is a scent you meet as you pass the bottle, with the far corners untouched. If the whole room is the brief, our guide to electric oil diffusers by stated room size starts from the floor area, not the bottle.

Evaporation, dust and how long a big bottle lasts

With the same reeds in the same room, a bottle lasts roughly in proportion to its volume: twice the liquid, somewhere near twice the time. There is no fixed figure, because warmth, airflow and the base in the liquid change the pace.

A bottle that stands for a long time collects dust on the reed tips and the liquid, and dusty reeds wick less. The light base evaporates ahead of the fragrance, so the last part can smell heavier than the first. Swap the reeds partway through, not only at the end.

Cost per month, large bottle against small

Per millilitre, a large bottle is usually the cheaper way to buy the same liquid, because the glass, stopper and reeds are a bigger share of a small bottle's price. The room decides the monthly spend, though: a neck full of reeds uses liquid faster, and the saving per millilitre goes on volume.

The comparison that matters is reeds against a machine for the room you have. Our comparison of what a bottle of oil or a pod costs per month rests on the makers' stated figures.

When an electric diffuser with a timer covers the room better

A reed bottle gives the same slow trickle day and night and waits for the air to come to it. A large or open-plan room wants scent carried across it while people are in, and nothing at night. An electric diffuser with a timer does that: a mist or a fan pushes the oil out, and the timer sets the hours.

Stay with a big bottle when there is no socket and the room is a hallway or landing; move to a machine when the room is open-plan or a full neck of reeds has already fallen short. Which electric diffuser suits an open-plan room is the question our comparison answers, with the coverage and noise each maker states or leaves unstated.