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
Kalkulilo (Calculator)
Kalkulilo is a comfortable calculator program and offers several different modes.
$2.99
PRICE
$2.99
No in-app purchases
BUSINESS MODEL
Paid
No overall Grossing estimate available
RATING
★ 4.5
29 ratings · Top 15%
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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What's included15 files
A product spec, phased build plan, differentiation notes, and screen references.
- START_HERE
- AGENTS
- EVIDENCE
- SPEC
- BUILD_PLAN
- DIFFERENTIATION
- skills/
- screens/
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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 Kalkulilo (Calculator): map the core Productivity 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
Add features like scientific calculations or graphing.
Assumption to check: Limited functionality compared to advanced calculators.
Validate with people who use this workflow before building.
Hypothesis 2
Implement customizable themes and layouts.
Assumption to check: No customization options for themes or layouts.
Validate with people who use this workflow before building.
Hypothesis 3
Introduce memory recall and storage features.
Assumption to check: No memory functionalities for storing values.
Validate with people who use this workflow before building.
Hypothesis 4
Enable support for fractions and more advanced mathematical functions.
Assumption to check: Lack of support for complex calculations (e.g., fractions).
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
Kalkulilo is a comfortable calculator program and offers several different modes. - The scientific mode offers all the usual functions and operations like the basic arithmetical operations, power, logarithm, roots, trigonometry etc. - The bit/integer mode offers logical operations (AND, OR, XOR, >>, etc.) on bit level. All values can be displayed as binary, octal, decimal or hexadecimal numbers. Calculations can be done in 8, 16 32 or 64 bits. - In the statistics mode you can create a sample by adding several values and then calculate mean, median, quantil values, variance, standard deviation, range etc. - Trigonometry mode provides lots of trigonometric functions, including legacy ones - The base/radix mode allows you to do calculations with numbers of base 2 to base 36 - The Time mode lets you directly work with time values (hours, minutes seconds) - A simple calculator is also included, with just the basic operators and large buttons. The user interface can be customized, you can change the key colors for different categories of functions and operators which helps to distinguish and find these more easily. The App also includes calculator widgets for the homescreen which allows to do calculations directly from your homescreen and a calculator for the Apple Watch.
Written user reviews are not shown here. Check current App Store reviews >
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
React Native
Expo
Zustand
React Navigation
React Native Gesture Handler
TypeScript
BUILD KIT DIFFICULTY
Intermediate
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.
