Teaching percentages with PhET (and what's missing) | No Drama Koala Maths

PhET is brilliant for fractions, decimals, and number sense. When it comes to percentages, there's a gap, and pretending there isn't is the fastest way to lose a parent's trust.

This post is about how to use PhET for percentages anyway, what it can and can't do, and where to reach for a different tool when the sim runs out of road.

What PhET does have

There's no PhET sim called "Percentages." There are a few sims that touch percentages in passing:

Ratio and Proportion — useful for percentages framed as ratios (eg. 30:100, 45:100). The "challenge" mode does support percentage display.
Plinko Probability — outcomes shown as decimals and percentages of total. Good for percentage-as-likelihood framing.
Number Line: Decimals — every decimal between 0 and 1 has a corresponding percentage (0.25 = 25%). The conversion isn't displayed automatically but the visual mapping is exactly the right idea.

That's the inventory. Not nothing, but not enough to teach percentages start to finish.

How to bridge the gap

Even without a dedicated sim, the PhET tools can carry a percentages lesson if you anchor it correctly. Two angles work well.

Percentage as "out of 100"

The Ratio and Proportion sim shows two colour bars whose heights are linked by a ratio. Setting the left bar to a fixed scale of 100 turns the right bar into a direct percentage display. If the right bar is at 30, that's 30%. Tilt the ratio and watch the percentage move.

This is the simplest visual mapping: percentages are ratios to 100. Spending ten minutes here, particularly with the challenge mode, locks the mental picture in.

Percentage as decimal in disguise

Pair Number Line: Decimals with this question: "if a decimal is just a position on the number line between 0 and 1, what would the position be for 25%? For 50%? For 99%?"

The conversion 0.25 = 25% stops being a rule to memorise and becomes the same point, described two ways.

This bridge is useful preparation for percentage increase/decrease problems later, where students who only ever learned percentages as "the % button on the calculator" tend to fall apart.

Where PhET runs out of road

Three big gaps you'll want to know about:

Percentage change problems. The "a shirt was $80, now it's $60, what's the percentage discount?" family of questions isn't in any PhET sim. These need worked examples and adaptive practice. Mathspace covers them with step-by-step working.
Percentage of a quantity. Calculating 35% of 240 isn't supported. Mathigon's Polypad can model this with fraction tiles, and an old-fashioned bar model (drawn on paper) often works just as well.
Compound percentages. Interest, repeated reductions, percentage on percentage — none of this lives in PhET. Spreadsheet exercises tend to be the best free option here.

For a complete percentages unit you'd combine the PhET sims above with adaptive practice (Mathspace handles the calculation side properly), worked examples (Khan Academy's percentage videos are solid), and a teacher reviewing where the student is actually getting stuck.

How we use these in the weekly review

In the No Drama Koala program, percentages tend to show up two ways.

The first is the bridge work — when a Year 7 student is solid on fractions and decimals but hasn't connected the third side of the triangle, I'll send a link to Ratio and Proportion with a specific prompt: "set the left bar to 100. What does the right bar tell you?" Five minutes there usually does the trick.

The second is misconception spotting. When a student's Mathspace data shows them confidently calculating 50% of something but flailing on 35%, the gap isn't percentages. It's the move from "easy benchmark" to "any percentage." A Loom video walking through the calculation as a fraction operation (35/100 × 240) usually closes the gap faster than another worksheet.

Try this with your child

If your child is in Year 7 to 9 and you want a quick check on whether their percentage thinking is connected to their fraction thinking, try this:

Open Number Line: Decimals on their device.
Ask them to drag a point to where 25% would be.
Then ask them what fraction that is.

If they put the point near 0.25 and say "a quarter" or "1/4" without prompting, the connection is there. If they hesitate, get stuck on "where does 25 go on a 0-to-1 line?", or put it near 0.025, the gap is worth fixing. It's not unusual. Schools often teach decimals, fractions, and percentages as three separate topics rather than three views of the same idea.

Want a qualified Australian teacher reviewing your child's actual percentages work each week and setting next week's tasks calibrated to their specific gaps? That's exactly what the weekly review program does. Start with a free discovery check-in, I'll send back a personalised video diagnostic of where they're at.