Methodology
Transparency is a core principle at PerGram. This page explains exactly how we collect, process, and present supplement pricing data. If something looks off, we want to know — use the Feedback button in the navigation.
Price Collection
We collect prices from major supplement retailers using automated web scrapers. Our scraper architecture is designed for accuracy and freshness:
- •Multiple extraction strategies — we use structured data (JSON-LD), retailer APIs, and HTML parsing as fallbacks to ensure we capture the correct price
- •Sanity checks — every price update is compared against the product's price history. Changes above 50% are flagged for review, and changes above 500% are automatically blocked until verified
- •Category-aware bounds — each supplement type has price floor and ceiling thresholds calibrated from real catalog data, preventing clearly erroneous prices from entering the system
- •Regular refreshes — prices are refreshed automatically with stale-product detection that prioritizes products overdue for updates
Supplement Facts Extraction
Extracting accurate ingredient data from supplement facts labels is one of the hardest problems we solve. Our multi-stage pipeline:
- HTML parsing — we first attempt to extract ingredient names, amounts, and units from the structured HTML on the product page
- AI vision analysis — when HTML parsing fails or produces low-confidence results, we use AI image analysis on the supplement facts label image to extract ingredients
- Confidence scoring — each extraction receives a confidence score based on data completeness and consistency. Low-confidence extractions are routed to a review queue
- Human verification — a review queue lets us compare extracted data against the original label image and correct any errors
Normalization
Normalization is the process of converting diverse product data into comparable metrics.
Unit Conversion
All ingredient amounts are converted to grams. Milligrams, micrograms, and IU (International Units) are converted using standard conversion factors. For probiotics, we use CFU (Colony Forming Units) as the standard unit instead.
Active Ingredient Identification
For single-ingredient products, we identify the primary active ingredient automatically. For blends with multiple active ingredients, we track the primary ingredient for the product's category (e.g., EPA+DHA for fish oil products in the omega-3 category).
Herbal Extract Ratios
Herbal supplements often list extract ratios (e.g., “10:1 extract”). We account for these ratios when calculating equivalent raw amounts, so extract products are compared fairly against raw herb powders.
Cost Calculation
The final cost per gram is calculated as: package price divided by (dose per serving × servings per container), converted to grams. We also compute cost per serving and cost per day for supplements with standard daily dosages.
Product Grouping
Many supplements are sold in multiple sizes or by multiple retailers. We use fingerprinting to group variants of the same product together:
- •Same brand + same active ingredients = grouped as variants
- •Brand name normalization handles variations like “NOW Foods” vs “Now Foods Supplements”
- •The best-value variant is highlighted automatically in each group
Ranking Independence
Product rankings on PerGram are determined solely by normalized price data. Rankings are never influenced by:
- Affiliate commission rates — a product with higher commission does not rank higher
- Paid placements or sponsored listings — there are none
- Brand partnerships — we have no brand relationships
PerGram earns revenue through affiliate links when you purchase a product through our site. This is disclosed on every page. See our full affiliate disclosure.
Known Limitations
No data system is perfect. Here are the limitations we're transparent about:
- Price lag — there can be a delay between a retailer changing a price and our system detecting it. We prioritize freshness but cannot guarantee real-time accuracy.
- Retailer coverage — we currently cover Amazon, iHerb, Swanson, and BulkSupplements. Some brands may be available at retailers we don't yet track.
- Sale prices — temporary promotions and coupon codes may not always be captured by our scrapers.
- Bioavailability — per-gram pricing treats all forms equally. In reality, some forms (e.g., chelated minerals) may offer better absorption per gram.
Last updated: April 2026