Question

calculating ratios for prices for the soap recipes is similar to doing stoichiometric calculations

Answer 1

Yes — that analogy is a good one. Calculating soap recipe costs works the same way as stoichiometry: you convert recipe “parts” or percentages to actual masses, use per-unit costs like a “molar mass,” and propagate those quantities through the recipe to get total cost and cost per unit. Below is a short explanation, formulas, a worked example, and practical tips.

How the analogy maps
- Recipe ratios / weight % = stoichiometric coefficients (relative amounts of each reactant).
- Batch mass (total grams) = the total mass of reactants you choose (like total moles × molar mass).
- Ingredient mass = recipe fraction × batch mass (analogous to converting moles → grams).
- Cost per gram (price/package weight) = analogous to molar mass for converting quantity to mass/value.
- Ingredient cost = ingredient mass × cost per gram (like mass × molar mass gives grams).
- Sum of ingredient costs = “total product” cost; divide by number of bars to get cost per bar.
- “Limiting reagent” idea = practical limits: an ingredient shortage or an ingredient that dominates cost (e.g., a pricey essential oil) will determine your minimum price or how you reformulate.

Basic formulas
- cost_per_g_i = price_i / package_weight_i
- mass_i = fraction_i × batch_mass (or mass_i = parts_i / sum(parts) × batch_mass)
- cost_i = mass_i × cost_per_g_i
- batch_cost = sum(cost_i) + overhead (packaging, labels, utilities)
- cost_per_bar = batch_cost / number_of_bars
- sale_price = cost_per_bar × (1 + desired_margin)

Worked example
Recipe (batch_mass = 1000 g oils total):
- Olive 40% = 400 g; price $10.00 / kg => cost_per_g = $0.010
- Coconut 30% = 300 g; price $6.00 / kg => cost_per_g = $0.006
- Palm 30% = 300 g; price $5.00 / kg => cost_per_g = $0.005

Assume lye needed = 136 g; lye price $3.00 / kg => $0.003/g. Water cost negligible here.

Costs:
- Olive: 400 g × $0.010 = $4.00
- Coconut: 300 g × $0.006 = $1.80
- Palm: 300 g × $0.005 = $1.50
- Lye: 136 g × $0.003 = $0.408
Batch ingredient cost = $7.708

Add packaging and labels, say $0.25/bar, and batch yields 10 bars:
- Overhead per batch = $0.25 × 10 = $2.50
- Total batch cost = $7.708 + $2.50 = $10.208
- Cost per bar = $10.208 / 10 = $1.0208
- Add 50% margin → sale price ≈ $1.53 per bar

Practical notes and adjustments
- Always use weight (grams) for accuracy; soap recipes are by weight, not volume.
- Account for superfat: lower lye required if you plan a superfat (e.g., 5% less lye). Use SAP values to compute exact lye weight.
- Include yield losses: trimming, failed batches, curing weight loss (water loss), and waste when determining effective cost per bar.
- Bulk purchases reduce cost_per_g; model different package sizes in your spreadsheet.
- Use a spreadsheet: store SAP values, % of each oil, package prices, and let it compute automatically when you change batch size or margins.
- Remember extra costs: electricity, rent, labor, shipping, taxes — these often make up a large part of final price.

If you want, I can:
- Build a template spreadsheet with formulas you can use, or
- Walk through the SAP/lye calculation step for a specific recipe including superfat.