How to Fold Filter Paper for Clean, Even Filtration

What You’ll Need

A circular filter paper of a size suited to your funnel
A clean funnel and the receiving vessel
The liquid you will use to wet the paper, usually the same solvent or liquid being filtered
A clean, dry work surface and clean hands or suitable clean forceps

Folding filter paper into a cone makes a supported, porous surface that fits inside a funnel. The liquid passes through while suspended solid particles are retained, though the paper’s grade and the setup affect how quickly and finely it filters. This guide covers the common cone fold for gravity filtration. Follow your lab’s procedure if it specifies a different fold, paper, or filtration method.

Step-by-Step

Check the paper and funnel. Choose a circular paper that will fit the funnel when folded: the cone should sit inside without its rim rising above the funnel rim. Check the paper for tears, holes, creases, or contamination. Make sure the funnel and receiving vessel are clean and stable.
Fold the circle in half. Align the edges carefully and crease across the diameter. Press the crease firmly enough to hold its shape, but avoid rubbing or tearing the paper.
Fold it in half again. Bring the two ends of the semicircle together to make a quarter-circle wedge. Match the curved edges as closely as possible so the layers will open evenly.
Separate one layer. Hold the folded wedge with the closed point at the bottom. Gently pull one outer layer away from the other three layers. Open it into a cone with one layer on one side and three layers on the other. Do not pull apart the paper at the tip.
Shape the cone. Open the paper carefully until it forms a hollow cone. The point should be closed and the top edge should be roughly level. If the cone will not open smoothly, ease the folds apart rather than forcing them.
Set it in the funnel. Place the cone inside the funnel, with its point resting at the funnel’s outlet. Adjust the paper so it follows the inside surface and does not stick up over the rim. Hold it in place while wetting.
Wet and seat the paper. Add a small amount of the liquid being filtered, or the solvent specified by your procedure. Wet the whole paper evenly and press it gently against the funnel wall so it stays in place. Check that there are no gaps or tears, then begin pouring slowly.

Common Mistakes

Opening the wrong number of layers: A standard cone has one layer on one side and three on the other. Separating the wrong layer can make an uneven or awkward shape.
Using paper that is too large: Paper extending above the funnel can spill liquid over the edge or fold inward. Choose a smaller circle or a funnel that fits it.
Leaving the paper dry: Dry paper may not cling to the funnel, leaving channels where liquid can run around the filter. Wet it evenly before adding the sample.
Making a hole at the point: Pulling the layers apart too forcefully can split the tip. Handle the paper gently and replace it if a tear goes through the filtering area.
Pouring too quickly: A fast stream can overflow the cone, dislodge it, or disturb the collected solids. Add the liquid in controlled portions and keep the level below the paper’s rim.
Touching the filtering area unnecessarily: Hands, dirty tools, or a dusty surface can contaminate the paper or sample. Handle the outside edges and use clean equipment.

Tips

Match the filter paper to the funnel and the purpose of the filtration. Paper grades differ in pore size, thickness, and flow rate; use the grade specified for your task.
Keep the folded point aligned with the funnel outlet, but do not push the paper so firmly into the stem that it blocks the flow.
Wet the paper with the liquid that is compatible with the sample and the procedure. Water is not suitable for every solvent or process.
For gravity filtration, keep the receiving vessel positioned so the funnel stem touches its inner wall when appropriate; this can help the filtrate run smoothly and reduce splashing.
If the procedure calls for a different arrangement, such as a pleated filter, do not substitute the cone fold. The fold affects contact with the funnel and filtration speed.
Dispose of used filter paper and collected solids according to the material and your workplace or laboratory’s instructions.

Troubleshooting

The cone keeps springing open: Recheck the folds and make sure the curved edges are aligned. Seat the cone in the funnel, then wet it evenly; a damp paper usually holds its shape better.

Liquid runs around the paper: Stop pouring and check whether the paper is the right size, fully seated, and wetted against the funnel wall. Reposition it or replace it if it is creased badly or torn.

Filtration is very slow: The paper may be too fine for the task, the liquid may contain a large amount of solid, or the paper may be clogged. Do not poke or scrape the filter; follow the procedure for changing paper, using a coarser grade, or filtering in portions.

The paper tears or the tip leaks: Stop and replace it with an intact circle. Transfer the sample carefully if possible, then check that the funnel outlet is not sharp or damaged and avoid forcing the cone into the stem.

The paper rises or slips while pouring: Pause, lower the liquid level if needed, and press the wetted paper gently back against the funnel. Pour down a glass rod or along the funnel wall if your procedure allows it.

Frequently Asked Questions

Which side of filter paper faces the funnel?

For ordinary circular laboratory filter paper, there is generally no designated front or back. Fold it into a cone and seat it evenly; follow the product or procedure instructions if the paper is specially treated or marked.

Why fold the paper in half twice?

The two folds create four layers that can be separated into a cone with one layer on one side and three on the other. This gives the paper a shape that fits a standard funnel.

Should I wet filter paper before filtering?

Usually, yes. Wetting helps the paper adhere to the funnel and reduces gaps around its edges. Use a liquid compatible with the sample and specified by the procedure.

Can I reuse filter paper?

Filter paper is generally intended for one use. Reusing it can carry over residue or contaminants and may change how it filters. Use fresh paper unless an approved procedure specifically says otherwise.

How do I fold filter paper for a coffee filter?

Use the fold recommended for the coffee maker. Many cone-shaped paper filters come pre-creased: open the seam folds as directed and fit the filter into the basket. A laboratory cone fold may not fit a coffee maker properly.

Does the same fold work for vacuum filtration?

Not necessarily. Vacuum filtration commonly uses a flat circular paper cut or selected to fit the perforated support in a Büchner-style funnel. Follow the equipment and procedure instructions rather than using the gravity-filtration cone fold.

Sources: Royal Society of Chemistry. Menus and buttons can differ slightly by version. Spotted something out of date? Tell us.