Product Specification: Free Online One Rep Max Calculator

Overview

This single-file application is a professional-grade strength training utility that estimates a user's one-rep maximum (1RM) based on submaximal weight and repetition inputs. It supports the two most popular strength formulas: Epley and Brzycki. The design prioritizes clarity, ease of use, and a clean, light-mode interface suitable for gym environments.

Core Features

1. Dynamic Calculation Engine: Instant calculation of 1RM as the user types or adjusts inputs.
2. Formula Selection: Toggle between Epley (1RM = Weight × (1 + Reps/30)) and Brzycki (1RM = Weight / (1.0278 - (0.0278 × Reps))).
3. Intensity Percentage Table: Automatically generates a comprehensive table showing 50% through 100% of the calculated 1RM (in 5% increments), helping users plan accessory work, warm-ups, and training blocks.
4. Unit Agnostic: Support for both Imperial (lbs) and Metric (kg) systems.
5. Responsive Input Form: A clean interface that works perfectly on mobile devices for usage while training.

UI/UX Specification

• Layout:
  • Header: Clean, centered H1 title followed by a brief, non-intrusive instruction sub-text.
  • Main Input Card: A centered, shadow-lifted card containing two number inputs (Weight, Reps) and a formula toggle dropdown.
  • Results Area: A clear, high-contrast display of the estimated 1RM.
  • Table Section: A responsive table below the result displaying training percentages.
• Aesthetics:
  • Color Palette: White background (#FFFFFF), soft light grey card surface (#F8FAFC), primary brand blue for buttons (#2563EB), and dark slate for text (#1E293B).
  • Typography: Sans-serif, modern font (Inter or system-stack) with high legibility.
  • Animations: Smooth opacity transitions for results appearance; subtle hover scaling on buttons.

Technical Directives & Constraints

• Architecture: The app MUST be a single HTML file. CSS and JS must be embedded. Use Tailwind CSS via CDN for styling.
• Storage: STRICTLY FORBIDDEN to use localStorage, sessionStorage, cookies, or IndexedDB. All variables must be ephemeral.
• Responsiveness: Use CSS Flexbox/Grid to ensure the layout stacks vertically on mobile and expands to a max-width of 600px on desktop.
• Safety: Ensure input sanitation for the math logic. Prevent division by zero if the user inputs 0 reps.
• No Popups: Do not use alert() or confirm(). If an error occurs (e.g., inputting 0 weight), show a visible, friendly inline error message within the DOM.
• Accessibility: Ensure high contrast ratios and appropriate ARIA labels for all inputs.