Why You Keep Forgetting Formulas

In the 1880s, German psychologist Hermann Ebbinghaus made a disturbing discovery. After learning new information, people forget 50% within 30 minutes and up to 80% within 24 hours.

He called this the Forgetting Curve - and it explains exactly why cramming the night before an exam feels productive but barely works.

Here’s the thing: this isn’t a flaw in your brain. It’s your brain being efficient - it dumps information it thinks you don’t need. Your job is to signal: “Hey, I actually need this.”

That signal is spaced repetition.

What is Spaced Repetition?

Spaced repetition is dead simple: instead of reviewing a formula once and hoping for the best, you review it at increasing intervals - just before you’re about to forget it.

Each review resets and flattens the forgetting curve, making the memory last longer every time.

Review	When	Memory Strength
1st review	Same day (within hours)	Lasts ~2 days
2nd review	Next day	Lasts ~1 week
3rd review	After 3-4 days	Lasts ~2-3 weeks
4th review	After 1-2 weeks	Lasts ~1-2 months
5th review	After 1 month	Nearly permanent

Spaced Repetition vs Cramming

	Cramming	Spaced Repetition
Time spent	3 hours in one night	5 x 10 minutes over a month
Feels like	Productive and intense	Easy (maybe too easy?)
After 1 day	Remember ~25%	Remember ~90%
After 1 month	Remember ~5%	Remember ~80%
Best for	Passing tomorrow’s test	Actually learning math

Research shows spaced repetition produces 25% higher retention compared to massed practice (cramming) over periods of 4 weeks or longer. And for math, where every new topic builds on previous ones, that long-term retention is everything.

The Spaced Repetition Method for Math Formulas

Here’s a practical system you can start today. You’ll need a stack of index cards (or an app like Anki).

Step 1: Create Formula Cards

For each formula, make a card with:

Step 2: Sort Cards Into Boxes (The Leitner System)

This is a simple, no-app-needed method that automatically spaces your reviews:

Step 3: Review Daily (Just 10 Minutes)

Here’s your daily routine:

1. Pull out cards due for review (check which boxes are due today)
2. Read the front - attempt the problem without flipping
3. Check your answer against the back
4. Sort: correct = move up, wrong = back to Box 1
5. Add any new formulas from today’s class to Box 1

Spaced Repetition for Different Math Levels

PSLE Students (P5-P6)

Your formula load is growing fast. Here are the key formulas to space:

Topic	Key Formulas to Card
Area & Perimeter	Triangle, parallelogram, trapezium, composite figures
Volume	Cube, cuboid, water tank conversions (1 litre = 1000 cm$^3$)
Percentage	Finding %, increase/decrease, finding the whole
Speed	$S = \frac{D}{T}$, $D = S \times T$, $T = \frac{D}{S}$, unit conversions
Ratio	Total given, difference given, changed ratio
Circles	$C = \pi d$, $A = \pi r^2$, semicircle, quarter circle

Pro tip for PSLE students: Make separate cards for each type of problem, not just each formula. For example, “Ratio: total given” and “Ratio: difference given” should be different cards because they require different approaches.

O-Level Students (S1-S4)

Your formula bank is much bigger. Prioritize formulas you keep getting wrong:

Topic	Key Formulas to Card
Algebra	Factorisation identities, completing the square, quadratic formula
Trigonometry	SOH-CAH-TOA, sine/cosine rule, area = $\frac{1}{2}ab\sin C$
Coordinate Geometry	Distance, midpoint, gradient, $y = mx + c$
Mensuration	Cone, sphere, pyramid surface area and volume
Probability	P(A or B), P(A and B), tree diagrams

The Secret Weapon: Interleaved Practice

Here’s something most students get wrong: they practice the same type of problem 10 times in a row. This is called blocked practice, and research shows it’s far less effective than interleaved practice.

In a classroom study, students who used interleaved practice scored 61% on a delayed test, compared to just 38% for those who used blocked practice. That’s a massive difference.

Why does mixing topics work? Because in an actual exam, you don’t get 10 ratio problems in a row. You need to identify which formula to use - and interleaved practice trains exactly that skill.

Your Weekly Spaced Repetition Schedule

Here’s a ready-to-use weekly plan:

Monday to Friday (10 min/day)

Time	Activity
Start of study session	Review due flashcards (Leitner boxes)
During practice	Mix problem types (interleaved practice)
End of study session	Add any new formulas to Box 1

Saturday (20 min)

• Do a formula brain dump: write every formula you can remember on a blank sheet
• Check against your card collection
• Any formula you forgot goes back to Box 1
• Try 5-6 mixed problems from different topics

Sunday (Rest or Light Review)

• Quick flip through Box 4 and Box 5 cards only
• No pressure - just keeping connections alive

Common Mistakes Students Make

1. Reviewing Too Often

If you review a formula every single day even after you know it well, you’re wasting time. The whole point of spaced repetition is to increase the gap between reviews. Trust the system - let the intervals grow.

2. Passive Flipping

Don’t just read the front of the card and immediately flip to check. Actually solve the problem on the front before checking. The effort of retrieval is what strengthens the memory.

3. Making Cards Too Complicated

Each card should test one thing. A card with 5 formulas on it defeats the purpose. If you can’t answer in under 30 seconds, the card is too complex - break it into smaller pieces.

4. Giving Up After a Week

Spaced repetition feels slow at first. You’ll wonder if it’s working. Research shows the benefits really kick in after 2-3 weeks of consistent practice. Stick with it.

Digital vs Physical: Which is Better?

Both work. Pick what you’ll actually stick with.

Method	Best For	Tools
Physical cards	Students who like hands-on learning; P5-P6 students	Index cards + a shoebox with dividers
Anki (app)	Students with lots of formulas; O-Level students	Free app with built-in spaced repetition algorithm
Quizlet	Students who want pre-made decks	App with community-created flashcard sets

How Spaced Repetition Saved 10 Minutes Per Exam

Here’s what happens when formulas are truly locked into long-term memory:

• No more “formula panic” at the start of the exam
• Faster problem-solving because you’re not wasting time trying to remember formulas
• More time for checking because you finished faster
• Less exam anxiety because you know that you know

Students who use spaced repetition report spending approximately 10 fewer minutes searching for formulas during exams - that’s 10 extra minutes for solving and checking.

Start Today: The 5-Card Challenge

Don’t try to card every formula you’ve ever learned. Start with just 5:

1. Pick the 5 formulas you forget most often
2. Create a card for each one (front = mini problem, back = formula + solution)
3. Put them all in Box 1
4. Review them every day this week
5. Next week, add 5 more

By the end of the month, you’ll have 20+ formulas in your system, many already promoted to Box 3 or 4.

Key Takeaways

• The Forgetting Curve is real: without review, you lose 80% of what you learn within 24 hours
• Spaced repetition fights back: reviewing at increasing intervals makes memories nearly permanent
• 10 minutes daily beats 3 hours of cramming: consistency is everything
• Use the Leitner Box System: a simple 5-box method that automatically spaces your reviews
• Interleave your practice: mix problem types instead of doing 10 of the same kind
• Start with just 5 cards: build the habit before building the collection