Wellhead Pressure Converter: Gradient, Depth, Formation P & API

Wellhead Pressure Converter

Pressure ↔ gradient ↔ depth · formation pressure · multi-fluid column

Fluid Gradient (★ custom)

Edit any one — the other two update. SG = gradient ÷ 0.433, ppg = gradient ÷ 0.052.

Solve (leave ONE blank)

Fill any two of pressure, gradient and depth — leave the unknown blank and it will be solved.

To solve for gradient instead, clear the gradient field above and fill both pressure and depth.

Surface → Formation Pressure

Fluid Segments (top → bottom)

Each segment has its own gradient and depth interval — e.g. completion fluid above a packer, heavier mud below.

API Gravity ↔ Specific Gravity

SG = 141.5 ÷ (131.5 + °API). Water is 10 °API by definition; lighter oils have higher API and lower SG.

Result

Fill any two of pressure, gradient and depth to solve the third — or switch modes to turn a shut-in surface reading into formation pressure, build a multi-fluid column profile, or convert API gravity to specific gravity

Every relation here uses true vertical depth — measured or along-hole depth in a deviated well gives a hydrostatic figure that is too high, which is one of the most consequential mistakes in pressure work. Gradients are treated as constant over each interval, which is reasonable for liquids but a simplification for gas, whose density and therefore gradient change substantially with pressure and temperature; a gas column shut in at surface can show a surprisingly small drop from bottom-hole pressure for exactly this reason. Reference gradients are typical values only, and formation-water salinity, oil composition and temperature all shift them, so measured or lab-derived values should always take priority. Shut-in surface readings must be stabilised, and a reading influenced by trapped pressure, a plugged float or migrating gas will not represent formation pressure. Use these conversions for engineering estimation and cross-checks, not as a substitute for well-specific pressure data or approved well-control procedures.

Wellhead Pressure Converter: Pressure, Gradient, Depth, Formation Pressure and API Gravity

A surface gauge reading on it’s own doesn’t tell you much, you always need to tie it back to depth and fluid gradient before it means anything real downhole. This hydrostatic pressure calculator starts with a genuine 3-way solver, give it any two of pressure at depth, fluid gradient and true vertical depth and it works out the third, so the same tool finds the pressure a column exerts, backs out an unknown gradient from a measured pressure or finds the depth where a certain pressure occurs. As a proper pressure gradient calculator, gradient can be typed as psi/ft, specific gravity or mud weight in ppg, edit any one field and the other two update automatically. A Formation Pressure mode adds a shut-in casing or drill-pipe reading on top of the hydrostatic column, which is really what every well-control gauge reading is actually about. Because a real wellbore rarely holds just one fluid, Multi-Column mode sums up separate segments, say completion fluid above a packer and heavier mud below, into one cumulative profile. And since API gravity comes up constantly in this same line of work, an API gravity calculator is built right in too.

How to Use

This tool has four tabs: Solver, Formation P, Multi-Column and API ↔ SG.

Setting your units (shared across all tabs)
  • Select your Pressure Unit: psi, kPa, bar, or atm.
  • Select your Depth Unit: ft or m.
Setting your fluid gradient (shared by Solver and Formation P)
  • Pick a Reference Fluid from the dropdown, fresh water, seawater, formation water, typical oil or typical gas, to fill in a typical gradient. Or just type your own values directly.
  • Edit any one of Gradient (psi/ft), SG or MW (ppg) and the other two update on there own, since the tool keeps all three in sync using the standard 0.433 and 0.052 relationships.
Using Solver mode
  • Fill in any two of the three fields, Pressure at depth, the Gradient above and TVD and leave the third one blank.
  • To solve for gradient specifically, clear the gradient field above and fill both pressure and depth instead.
  • Tap Calculate to get the value you left out.
Using Formation P mode
  • Switch to the Formation P tab. Enter your Shut-in Surface Pressure (SICP/SIDPP), the stabilised reading off your gauge.
  • Enter your Target TVD.
  • Tap Calculate. The tool adds this surface pressure to the hydrostatic column above that depth, using the gradient you set up above, to give you formation pressure.
Using Multi-Column mode
  • Switch to the Multi-Column tab. Tap Add Segment for each distinct fluid interval in the wellbore, top to bottom, entering it’s own gradient and depth range, for example completion fluid above a packer, then heavier mud below it.
  • Enter a Surface Pressure (optional) if there’s any pressure sitting on top of the whole column.
  • Tap Calculate to get a cumulative pressure profile plus an equivalent average gradient for the whole column.
Using API ↔ SG mode
  • Switch to the API ↔ SG tab. Enter either your °API value or your Specific Gravity, whichever one you already have and leave the other blank.
  • Tap Calculate. Remember, water sits at 10°API by definition, lighter oils have a higher API number and a lower SG.
Exporting your result
  • Use Print / PDF for a clean printable copy or Copy to paste the numbers into your well file.

Key Features

  • Genuine 3-way bidirectional solver, any two of pressure, gradient and depth gives you the third
  • Gradient interchange, psi/ft, SG and ppg all stay synced, edit any one and the others follow
  • Formation pressure mode, turning a shut-in surface reading (SICP/SIDPP) into an actual formation pressure at depth
  • Multi-fluid column mode, summing separate gradient segments into one cumulative pressure profile with an equivalent average gradient
  • Built-in API ↔ SG converter, the companion conversion that always comes up alongside pressure work
  • Multiple pressure and depth units, psi/kPa/bar/atm and ft/m
  • Editable reference fluid list, fresh water, seawater, formation water, oil and gas, always overridable with your own measured value
  • Export as PDF or copy the result.

Formula / Logic Used

Hydrostatic Pressure (Solver Core)

P=Gradient×TVDP = Gradient \times TVD

Rearranged as needed to solve for whichever of the three values is left blank.

Gradient Interchange

SG=Gradient0.433,MW (ppg)=Gradient0.052SG = \frac{Gradient}{0.433}, \qquad MW\ (ppg) = \frac{Gradient}{0.052}

Formation Pressure

Pformation=Psurface+(Gradient×TVD)P_{formation} = P_{surface} + (Gradient \times TVD)

Multi-Column Cumulative Pressure

Pcumulative=Psurface+∑segments(Gradienti×ΔDepthi)P_{cumulative} = P_{surface} + \sum_{segments} (Gradient_i \times \Delta Depth_i)

The equivalent average gradient is then this total pressure divided by total depth, giving one single number that represent the whole mixed column.

API to Specific Gravity

SG=141.5131.5+°APISG = \frac{141.5}{131.5 + °API}

Water sits at exactly 10°API by definition, so anything above 10°API is lighter then water and anything below is heavier.

Who Should Use This Tool

Drilling and well-control engineers converting a surface gauge reading into a downhole pressure, or working out a mixed fluid column’s cumulative pressure. Also useful for petroleum engineering students learning gradient, SG and mud-weight interchange and the surface-to-formation pressure relationship, for there well control coursework.

Frequently Asked Questions (FAQs)

1. How do I convert a fluid gradient in psi/ft to specific gravity?

Divide the gradient by 0.433. This tool keeps gradient, SG and mud weight in ppg all synced together, so editing any one of the three fields updates the other two instantly using the standard relationships.

2. How does a shut-in surface pressure reading turn into formation pressure?

Add the shut-in surface pressure (SICP or SIDPP) to the hydrostatic pressure of the fluid column above the depth your interested in, formation pressure equals surface pressure plus gradient times TVD. This tool’s Formation P mode does exactly this once you enter your surface reading and target depth.

3. Why do I need a multi-column mode instead of just one gradient for the whole well?

A real wellbore very often holds more then one fluid at once, lighter completion fluid above a packer and heavier kill mud below it, for example, and using a single averaged gradient across both would give the wrong pressure at any specific depth. This tool sums each segment’s own contribution separately for an accurate cumulative profile and also gives you an equivalent average gradient for the whole thing.

4. Why does true vertical depth matter so much for all of these calculations?

Hydrostatic pressure depends purely on the vertical height of the fluid column, not the actual length of pipe or hole drilled to reach that point, so using measured depth on a deviated well overstates the real pressure, which is one of the more consequential mistakes in this kind of work. Every calculation in this tool is built around TVD specifically for this reason.

5. What does a higher API gravity number actually mean for an oil?

It means the oil is lighter, API gravity runs the opposite way to specific gravity, water sits at exactly 10°API, and oils lighter then water have a higher API number and a lower SG. This tool converts directly between the two using the standard 141.5/(131.5+°API) relationship.

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