How to Write a Lab Report (Structure, Sections, and a Full Example)

A lab report is not a story about a lab session. It is a record precise enough that someone else could repeat your experiment and get the same result, followed by an honest account of what your data does and does not show. Almost every mark you lose is lost by forgetting that, and writing a narrative of your afternoon instead. This guide walks through each section, what it is actually for, and the mistakes that cost easy marks.

What a lab report is graded on

It is rarely graded on whether you got the textbook answer. Experiments go wrong, and a report that honestly explains a result that came out too high, with a sensible reason, usually scores better than one that quietly rounds the data toward the answer it was supposed to get. Two things carry most of the marks: whether your method is complete enough to repeat, and whether your interpretation follows from the numbers you actually collected.

Read your own draft with one question in mind: could a student in the year below run this experiment from your report alone, and would they read your data the way you did? If not, that is where the marks are leaking.

The standard sections, in order

• Title: what was measured, in a specific line.
• Abstract: the whole report in about 150 words, written last.
• Introduction: the question and the relevant background.
• Methods: exactly what you did, so it can be repeated.
• Results: the data, presented, with no interpretation yet.
• Discussion: what the data means, and why it came out as it did.
• Conclusion: the short answer to the question you set.
• References: where the background and methods came from.

Not every course wants every section, and some merge Results and Discussion. Follow your own lab manual where it disagrees with this list, because the person marking it wrote that manual.

Methods: the section markers actually check

The Methods section is where reports are quietly won or lost, because it is the one part a marker can verify against what they know happened in the lab. Write it so the experiment could be repeated: concentrations, volumes, temperatures, times, equipment and settings, in the past tense. If you changed the protocol partway through, say so, because leaving it out is the difference between a record and a rough memory.

Do not copy the printed instructions and do not write it as a numbered list of commands. Instructions say "add 10 mL of the solution". A methods section says "10 mL of the solution was added". You are reporting what was done, not telling someone what to do.

Results: show the data, hold the opinion

The Results section presents what you measured: tables, graphs, and the sentences that point the reader to what is in them. It does not yet say what any of it means. That restraint is the point, because a marker wants to see the raw picture before your interpretation, so they can judge whether your interpretation is fair.

Every table and figure needs a number and a caption, and every one needs a sentence in the text that refers to it. A graph nobody points at might as well not be there. Report the units once, clearly, and keep the significant figures honest: a balance that reads to two decimal places does not justify a result quoted to five.

Discussion: where most of the marks live

The Discussion is where you interpret. It answers three questions in turn: what do the results show, why did they come out this way, and where might they be wrong. The strongest reports spend real space on the third one. Naming a specific source of error, and saying which direction it would push the result, shows you understand the experiment rather than just performing it.

Avoid the empty error sentence. "Human error may have affected the results" is worth nothing because it is true of everything and explains nothing. "The 30 second delay between mixing and the first reading would have let some product form before timing began, which would raise the apparent initial rate" is a real analysis, because it is specific and it says which way the number moved.

Conclusion and references

The Conclusion is short: the answer to the question in the Introduction, in a sentence or two, with the key figure. It introduces nothing new. If you find yourself explaining something in the Conclusion, it belonged in the Discussion.

Reference the sources you used for the background and any method you did not design yourself, in whatever style your department requires. Consistency matters more than the particular style: pick the one your course uses and keep every entry in the same shape.

Should you write in the passive voice?

This is the question every science student asks, and the honest answer is: follow your department, because conventions differ. Traditional lab reports use the passive in the Methods ("the solution was heated") to keep the focus on the procedure rather than the person. Some modern courses now allow the first person. Neither is wrong in general. What is wrong is switching between them at random inside one report, so decide before you start and stay consistent.

Common mistakes

• Retelling the session. "First we got the equipment, then we set up" is a diary, not a method.
• Interpreting in the Results. Save what it means for the Discussion.
• The empty error line. "Human error" with no specifics earns nothing.
• Copying the lab manual as your Methods, word for word, in the imperative.
• A figure with no caption, no number, or no sentence pointing to it.
• Quoting results to more decimal places than the instrument can measure.
• Adjusting data toward the expected answer instead of explaining the gap.
• An abstract written first, so it promises a report you did not end up writing.

The abstract mistake is worth a second look. Write the abstract last, once the report exists, so it summarises what you actually found rather than what you hoped to find. It is a summary, not a plan.

A last check before you submit

• Could someone repeat the experiment from your Methods alone?
• Is the Methods in the past tense, not a list of commands?
• Does the Results section show the data without interpreting it?
• Does every table and figure have a number, a caption, and a sentence pointing at it?
• Does the Discussion name at least one specific source of error and its direction?
• Does the Conclusion answer the question and introduce nothing new?
• Is the voice (passive or first person) consistent throughout?
• Was the abstract written last?

A lab report is one of the few pieces of writing where being plain is the whole skill. You are not trying to impress anyone with the prose. You are trying to leave a record clear enough that the experiment could happen again and honest enough that the reader trusts your reading of it. Do those two things and the marks follow.