Feature spec
Goal: Send a clear invoice after finishing a job.
- Add a client, line items, a due date, and payment instructions.
- Preview the total before exporting a PDF.
- Keep an editable draft if export fails.
App Overview
Calculator Plus - PRO
the world’s favorite calculator app, with more than 40 million downloads worldwide
$3.99 · In-app purchases
PRICE
$3.99
In-app purchases
BUSINESS MODEL
Paid
No overall Grossing estimate available
RATING
★ 4.9
695 ratings · Top 5%
Evidence scope: No current overall US chart position is available. Category-chart positions shown in discovery are a separate scope. Revenue is a directional model estimate, not verified earnings.
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Build Kit
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A product spec, screen plans, and build steps for your coding agent.
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Combine features and design from your favorite apps.
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What's included16 files
A product spec, phased build plan, differentiation notes, and screen references.
- START_HERE
- AGENTS
- EVIDENCE
- SPEC
- BUILD_PLAN
- DIFFERENTIATION
- skills/
- screens/
Preview a sample Build Kit No email needed
Illustrative example: an invoice app for independent contractors. Your kit is tailored to the app you choose; this is not its generated content.
Screen plan
Invoice editor: Client at the top, editable line items in the middle, total and preview action at the bottom.
States: Empty draft, validation errors beside each field, exporting, and a retry action that preserves the draft.
Differentiate and validate
Hypothesis: Contractors need faster repeat invoices more than more templates.
First experiment: Test duplicating a previous job with five contractors. Watch where they hesitate before expanding the feature set.
The full kit adds a phased build plan, evidence notes, tool prompts, and implementation guidance. It is a plan for your coding agent, not a finished app.
Usage: Mechanics are fair game; never reuse the original name, branding, assets, or verbatim copy.
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Free quick prompt For a smaller starting point
Quick prompt
Good fit for mobile-first iOS and Android prototypes with managed app scaffolding.
// RORK SYSTEM PROMPT
Rork plan preview for Calculator Plus - PRO: map the core Utilities workflow, choose the smallest differentiated feature set, define the data and monetization boundaries, then prototype the riskiest user journey first.
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Opportunities
AI-GENERATED OPPORTUNITY HYPOTHESES
These are generated suggestions, not verified review quotations. Check current App Store reviews and speak with users before treating a possible gap as a requirement.
Hypothesis 1
Provide tutorials or tooltips explaining advanced functions.
Assumption to check: Some users find the scientific calculator complex to use.
Validate with people who use this workflow before building.
Hypothesis 2
Allow users to choose themes or layouts.
Assumption to check: Limited customization options for the interface.
Validate with people who use this workflow before building.
Hypothesis 3
Implement a more subtle upgrade prompt after extended usage.
Assumption to check: In-app purchase prompts can be intrusive.
Validate with people who use this workflow before building.
Hypothesis 4
Introduce graphing capabilities in the pro version.
Assumption to check: No support for advanced features like graphing.
Validate with people who use this workflow before building.
App evidence
Original App Store material. Ratings and screenshots describe the existing app, not proof of demand for your version.
App Store description
the world’s favorite calculator app, with more than 40 million downloads worldwide. the perfect day-to-day calculator for both iphone and ipad, calculator plus is easy to use and beautifully designed to be your daily calculator. calculator plus makes math simple and fun with crisp hd and a layout for fast and easy calculations. solve all of your everyday calculations, from simple addition to fractions and multiplication. calculator plus remembers everything you calculate, and lets you review your history and past calculations anytime, making it perfect for shopping, doing homework and schoolwork, balancing checkbooks, making tips, taking notes, calculating taxes, and everyday math problems you’re sure to encounter. calculator plus is also an exceptional school calculator. calculator plus is the perfect calculator for teachers, students, and working professionals. if you quit the calculator and go do something else, it's all still there whenever you come back. you'll never need to type the same calculation twice again making it the perfect math calculator to take the best notes and solve any problem. features: - calculations in clear, elegant type that's easy to read - perform all the basic calculations you could ever need including addition, subtraction, division, multiplication, and percentages - use backspace anytime to correct a simple mistake, instead of starting over - keep track of your results using the memory buttons and history screen - express yourself with light and dark themes - for more complicated and advanced problems, use options like squaring, square root, percent and the pi button try out calculator plus – the simplest, strongest equation solver around! from everyday addition and subtraction to complex percent and fraction problems, you can solve and track it all for free. download calculator plus for your iphone or ipad and finally put that handheld calculator to rest. this version of calculator plus is ad-supported. an ad-free version is also available. calculator plus © 2022 digitalchemy, llc
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Screenshots
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Build scope
Plan a focused first version with your coding agent. These are planning assumptions, not a delivery guarantee.
Define the first version
Choose one audience and one core workflow. Use the kit to agree on its screens, data, and acceptance criteria before building.
Set boundaries
Decide which secondary features, integrations, and platform support can wait. Your version does not need to reproduce everything in the original.
Check the hard parts
Validate external services, specialist technology, data access, and ongoing costs for your chosen scope.
SUGGESTED BUILD STACK
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BUILD KIT DIFFICULTY
Starter
From the Build Kit assessment.
A reliable timeline needs an agreed scope and a technical check. Ask your agent to estimate the phases in BUILD_PLAN.md after that review.
